That's because they're less Rome and more "nomadic raiders with Roman drill and discipline". They're continually on the march, continually fighting or drilling, and while they may not use most chems, stuff like healing powder, the bitter drink, and hydra see extensive use among their ranks. They may not be Rome itself, but Gaius Marius would shed a single manly tear of appreciation at seeing how wonderfully the Legion campaigns. Caesar flat-out admits to the Courier the current Legion is just an end to a means for him, and as soon as he takes New Vegas he'll reform them into something truly Roman instead of what they are now. Its ironic really that the Legion are no different from the barbarians that conquered Rome in that regard.
And some cut content from Sawyer:
Long story short, he's almost certainly going to pause and reform and rebuild his army after conquering actual civilized areas. Remember, unless Meyer is Sheriff of Primm and gets them to resist, in a Legion victory it stays open as a trading post with constant Legion presence... not exactly what you would expect from the Legion as we see it, now is it? Add in the fact that the Legion is already capable of operating along modern lines with their current Roman-styled organization and hierarchy and you have the makings of a truly Roman military, armed and equipped with modern weapon and equipment. Especially if Caesar can "convince" the Gun Runners at their factory to provide for the Legion.
Remember, Caesar taught the Blackfoots small-unit tactics and firearms maintenance and care when he started out, so it isn't like he has some hate-boner for firearms. I've posted elsewhere in this thread about Caesar being ruthlessly pragmatic in how the Legion operates instead of stupid, so I won't repeat myself there again. Long story short, less about hating guns and more about logistics, resupply, and potential slave uprisings being a bitch. With control over Vegas and an expansion of merchants (see Dale Barton at the camp), all three of those issues are mitigated, so a modern-style reformation isn't exactly impossible, and almost certain to happen among the veterans.
Basically, the Legion has more in common with Augusto Pinochet and other third-world dictators than any barbarian tribe, as much as NCR fans would like to say otherwise. They patrol their territory and collect taxes, as well as kill troublemakers. So long as you nod your head and do as you're told, you're allowed to carry on with your business. And I'm sure that if these civilians contributed to the Legion in some way, they'd get rewarded by Caesar one way or another.
If I were to create say, Legion civilians in the Fort, I'd mostly have rich and smart folks running around in the camp paying tribute to Caesar. Some brahmin rancher paying tribute in the form of beef and other food goods for the army, some ex-Enclave guys who come by to "donate" newly-made plasma rifles and power armor to Caesar's elite guard, and I'd even have them hang around the slave pens after their "audience" with Caesar where they'll be buying newly-captured NCR slaves from some Legion troops auctioning them off for some money. The brahmin rancher will buy some freshly-enslaved NCR troops and civilians for some pack mules, the Enclave dudes will buy some captured NCR rangers for their science experiments, among other things.
Once Caesar conquers the Mojave and eventually, California, the Legion will partially resemble the NCR, and they might even employ female soldiers and use drugs in the future. Caesar might settle down and create the militarized republic he wanted Aaron Kimball to make, where basically it'll be similar to the NCR in most aspects, except it's under military rule. Caesar himself admired Kimball to a certain extent, only to be disappointed when Kimball became a politician instead of seizing control of the legislature with his army:
"A man of potential, held back by the craven political context he inhabits. You realize he was a general? 'The Hero of the Mojave,' they called him. A title he earned by extirpating lesser tribes that dared attack NCR citizens. His responses were swift and draconian. President Tandi, the 'founding mother,' coddled hostile tribes... but her successors were less naive, so they gave Kimball free rein. And after a respectable military career, what does he do? Become a politician. What better way to stunt the growth of leaders, not to mention whole cultures? A leader shouldn't have to kowtow to those who serve him. With so much energy wasted on those below, how is he ever to move forward? Had he taken the government by force, used his army to stage a coup, things would be very different. I'll just have to do it for him."
Considering that Caesar himself was an NCR citizen, turning the Legion into a militarized version of the NCR was probably going to be his endgame. Caesar admired Kimball for his decisive actions against hostile tribes, and he only turned against the man because Kimball became a political whore. Had Kimball taken control of the government by force with the military, Caesar would have probably allied with him instead. You'd get no Legion-NCR war, just two military dictators trading with each other in the west while hunting down what's left of the Western Brotherhood of Steel and using House as a go-between for both their factions.
So 20 years down the line, you'd see a more cosmopolitan Legion, one that has probably adapted the use of technology like power armor and riot gear, and one that would have some of the NCR's advantages, while having none of its weaknesses. It would also have the side effect of making the Legion more stable once he dies, the generals would probably just elect a new leader the moment he kicks the bucket. Someone like Vulpes or the Courier, perhaps.
