I think AAA games gain complexity from grafting many technologies together. Yes a 3D engine and GPU stuff adds complexity that factorio sidesteps. And also, to many folks here they see the complexity of factorio because they are exposed to it, while AAA games hide the game mechanics from the user.

However Factorio seems to have done everything from scratch. the game engine, the UI, etc.

And don't forget the (non-technical) complexity of developing out in the open with their blog and alpha testers.