There would be something stopping them. They couldn't call 1/3 of a civ a civ!*
Also, they couldn't extract money in the same way from a person who wants to play, through the whole game, a particular this-world civilization: by putting three chunks of it in three separate DLCs.
What they are likely to find, probably are presently finding, is that this is a dud of a pricing strategy because potential customers 1) see through it and 2) resent it.
Building resentment in your customer base is not a good way to make sales.
(Again that is IF the tail wagged the dog, which I'm not sure I believe. I think the game was mostly developed in good faith; I believe the designers convinced themselves that ages and civ switching would make for a fun game.** But, for all that, I'm sure there was also a meeting b/w the uppity-ups at Firaxis and the uppity-ups at 2K where someone said "and this will also lend itself to packaging civlets*** as DLC content")
*it's "shrinkflation." I buy K-pods for the office coffee-maker, but it's only about once a month that I need an extra cup of coffee, so it's been a year since I bought a multipack. When I brought the new multi-pack out to the office, I noticed it was ten pods as opposed to the earlier 12 pods. They now make an entity that plays for 1/3 of the game, but still call it a "Civ." If my K-pods had dropped from 12 to 4, I would have been way more disappointed/skeptical/angry.
**I think they thought that trying out different leader+civ combos (trying to find which combination of all those perks was most OP) would be engaging to players.
***except they didn't say "civlets" (because I hadn't made it happen yet).