Sure, the Pathfinder games don't have flight either, but people understand that it's because those are lower budget games. But people logically expected a lot more from BG3 in comparison.
I don't think flight implementations are an issue of money, it's an issue of complexity. Three axis turn based combat is tricky to get right, and it makes more sense to relegate flight to some kind of buff+animation than actually have some robust system where you can drop barrels from 60 feet in the air.
