What did you expect? They don't have most of their pops leading hedonistic lifestyle just so that you can force them to work their automatic buildings. Or, in case of Spiritualist FE, actually working their districts or creating consumer goods and amenities out of thin air.
Gameplay>Lore. However even on the lore front this makes no sense as they are meant to be
Stagnant empires, not just ones that decided at one point that this was good enough for them. There should be a huge number of unfilled jobs and empty housing on these enormous idyllic worlds, not just a planet full of complacent demi-gods that are satisfied with a fraction of their former glories. I guess it depends which FE you are looking at but some don't really fit their designation as fallen empires.
Your argument also makes no sense as the precursor pops do work jobs, the jobs just don't come from the buildings.
On the gameplay front these worlds are boring to own compared to regular worlds in 2.2 and are a pretty disappointing reward for the effort to acquire them.
Has anyone seen what FE's do with worlds once they Ascend? Do they just plop down more fire-and-forget buildings or are they a bit more proactive?