Logistically books are kind of a nightmare. In the first few editions it wasn't reasonable to expect people to have digital access to rules, so books were the only way you could do it. But now there's nothing stopping GW from releasing the codex as a free PDF that I can download. Books are expensive to produce, it's another thing I'd have to lug around, and I can't tap on a keyword to get a little popup with a rules summary. If you really want books as an optional release that's fine, but I don't know if GW can make it worth the time and money to produce them for the few people who want them. Better to make a book purchase required, even if you just want the code, so they can justify printing them. Maybe a codex is a more appealing purchase if a player knows that this book will be good for the next three years, but if I can pay a couple bucks for a PDF version that I can print at Staples if I really wanted to, then I would.
Seriously, this. It's fucking 2025, people have phones and the battery lasts more than an hour with the screen on.
No, I don't want to lug around a 300 page core rulebook. A 100 page codex. A 75 page narrative warzone book. A 40 page points book. 1-2 issues of White Dwarf, and then possibly another 80 page fluff book that has a unit sheet printed in the back of it. That's a bunch of bullshit to be stuck carrying around in a backpack like some schoolkid, 90% of which I actually don't even need but it's just stuck with the rest. And then on top of all of that, still need an army list and have to remember which rule/stratagem/datasheet/etc. is in which book if I have to actually reference it in the middle of a game.
And that's all ignoring being able to update anything. Dev wound spam eldar with a 75% win rate was bad enough at the start of 10th, not fixing that would have likely actually made a dent in the amount of people willing to play the game. Or the 9th edition Votann codex that was so fucking bad GW apologized for it before launch, nerfed all of it, rendering the back half of the book with the rules in it entirely useless for playing the game.
People either weren't playing, had things work out in their favor, luckily never encountered shit, or just plain forgot when armies didn't get updates for a decade at a time. Leaving things either broken and unplayable, or broken and overpowered. If I decide to drive 2 hours to play a game, I don't want to have to spend 30 minutes going over the local gentleman's agreements that are attempting to fix a busted mess that hasn't been updated in ages possibly to find out that the shit I did take to play with isn't considered playable there because that particular store's players have all collectively banned a specific character or something to avoid an issue that I may not even be aware of.
And sure, there are other games out there that do not get updated as often and don't need to be. But they're also usually smaller games with fewer factions or factions that aren't as distinct. Team Yankee and Bolt Action don't need to balance an equivalent of imperial guard, chaos daemons, and resurrecting necrons all within the same system. Even within Star Wars Legion things aren't as drastically different between armies.