Too many posibilities, professional pressure, emotional manipulation, maybe they are dangling some kind of reward in return to make the devs overextent themselves that much.
There's a certain direction we the consumers are pushing the AAA industry: We want bigger worlds, better graphics, improved AI, and by the many-folds. We don't want "a little bit" bigger maps, but WAY bigger maps. WAY more quests, WAY more dialog, we want LOTS of famous actors - etc etc
This means they have to pile on the resources, more more more work-hours, for design and building and artwork and programming and management and administration, paychecks paychecks paychecks, but - and here's the big but, we don't really want to pay much more for it.
Movies work similarily, they're huge investments, but - they get huge returns. "Everybody" watches movies. Far from "everybody" plays games, so you get an industry having to approach a movie kind of budget, but their target audience is a tenth of what movies have