Pretty much. I mean even during WW2 you had Willow Run, which was over three million square feet in size (325,160 square meters for you Eurofags here), which isn't exactly the sort of thing that can be picked up and moved, nor could the railyards that led to it, nor the component factories that used the rail lines to ship parts for final assembly... That Defiance Industries plant on Hesperus is probably close to the size of a small city if not larger, complete with dormitories, amenities, etc.
In the Novel "The Dying Time", several lances worth of mechs engage in battle
inside the Defiance Industries bunkers and there is enough room for them to get lost and only engage each other every now and then in ambushes... and this large-scale combat actually only affects a fraction of the facilities inside that mountain.
DefHes is insanely huge, has a lot of facilities spread all over the planet and it needs resources, material and components from other facilities, possibly lightyears away. In a way, DefHes sits in the middle of a vast star-spanning net of suppliers as well as a giant military apparatus that both defends them and depends on them.
Relocating all that is a massive undertaking, since it creates an endless chain of other stuff that needs to be relocated as well (which in turn will also make it necessary to relocate stuff to supply that newly relocated stuff and so on) and then you run into issues of supply chains running long and thin.
At some point you just settle for thicker bunker doors, figuratively speaking. Especially when this also allows you way shorter resupply chains to your frontlines.
Weathering a raid or two every couple years is not pleasant, but you just hunker down, keep your stocks in your vaults and sell your skin as expensive as possible, hoping for reinforcements to disperse the raiders before they actually break into your facilities and get anything of value.
I called it a flimsy excuse, but the more I think about the logistical nightmare that is relocating such vast facilities, the more reasonable it seems to just take the risk of raids and boost defenses instead.
My big problem is how, through centuries of relatively static borders, there are so many vitally important factories near constantly-active warzones. Like, the Great Houses have existed in some way or another since at least the 2300s. They've been in conflict or preparing for conflict with their neighbors for about that long. Even during the Golden Age that was the Star League, they kept building up their reserves. And yet seemingly no one thought of funding or relocating industries to worlds that wouldn't so easy to invade, capture or harass. Hell, the Lyran Commonwealth moved its capital from Arcturus to Tharkad, yet in 700+ years no one could convince Defiance to build their manufacture plants beyond the range of raids out of the Combine or the League.
You need to look further than the length of time and look at what was going on during that time.
In the early days, pre Star League, the necessary infrastructure to make a large scale production facility possible only existed within the heart of the Inner Sphere, cause those were the regions with the longest running colonies and settlements. You depend heavily on supplies of material, components and staff that you won't find easily anywhere else and you need to ship out your products too, so you'll end up somewhere close to Terra.
Once Star League did roll around, they kept a tab on such facilities and would discourage (if not outright forbidding) anyone from setting up facilities of similar scale too far away from the reaches of the SL. And with Star League, there were a couple good reasons to be close to the Terran Hegemony and not much risk involved in keeping it that way... and the facilities prospered and grew to absurd sizes, that make it virtually impossible to relocate them now.
After the Star League got screwed, things escalated rather quickly and the means to set up comparable facilities to those of DefHes would be gone within a few years, never quite to return until centuries later (if at all). The Succession Wars ate up any state's ability to relocate such vast facilities, let alone create them. If Succession States had the ability to create facilities on the level of DefHes from scratch, the entire IS would be covered in such facilities, so relocating existing assets is the best you can do, and it would be a pain in the ass:
It is by far more easy to relocate your capital, since that mainly involves moving people into new buildings and station garrisons around it. You really only need to create office space and communication systems... the worst thing here might be dealing with living quarters that are not as nice and luxurious as the ones you left behind.
Doesn't mean relocating your capital is a cakewalk, but putting a few bureaucrats and their paperwork into a couple DropShips and hauling them off a few dozen lightyears is way more easy than trying to hollow out an entire mountain's worth of underground Mech-production facilities and set up new shops in a place that might not even be capable of supplying the necessary raw materials and tech to restart production without building all that up first, which might in turn be an undertaking spanning generations.
And needless to say: That whole process of relocating those facilities would mean production ceases for quite some time.
You need to pack up all your stuff, all machinery, all your stored components, raw materials, partially finished products, everything. You need to ship it across the IS, do god knows how many runs back and forth to ferry everything to the new location and then hope everything made the journey without getting misplaced, broken or lost. Just one wrong barcode on a shipping container might mean you don't have a part of your assembly line and it'll take forever to find that container, dig up the stuff you need and continue with everything... and all this is assuming that you can even move everything that's important. Might as well be that some core aspects of the facilities and assembly lines in question are simply impossible to pack up and transport.
Silly example, but a friend of mine has a huge ancient fridge in his basement that was put there before the house was built on top. It is simply impossible to move that thing out of said basement, cause both stairways are too narrow.
So anyone planning to relocate his weapon production lines would need to spend absurd amounts of resources to prepare the new location and then wait (possibly for years!) until the new site ramps up production. And if anyone gets wind of any of this going on (and keep in mind, the planning alone could take DECADES, so plenty of time for someone to learn about it), this whole thing could become a desaster.
Just imagine someone successfully managing to intercept a jumpship carrying part of the DefHes haul. If it gets destroyed, it's bad, if it gets captured, it's even worse.
Putting together a protective naval force to accompany these hauls will leave every other area at risk of being attacked. It would slow down things even more.
Operation U-Haul would take years to be finished and production being at previous levels.
So yeah...
"We'll just pour all our economic surplus into relocating one of our military weapons production facilities, which will take decades to plan out and prepare for, which will mean production of those facilities will simply stop for a couple years during which we relocate everything and it has a decent risk of something going horribly wrong."
-vs.-
"Like... put another couple dozen turrets and a few minebelts around the factories, we'll double the size of the local garrison and keep a few battalions in close systems on standby."
