Why would I even want to build a game if I'm going to have to give between 4-8% of everything I make to steam and unity? I completely understand why nearly every AAA studio builds their own client and game engine, it's cheaper in the long run.
It depends on how much you're actually selling. Consider how much would it cost to hire devs to build your own custom engine? (Don't forget console support). Your game needs to pull in 20x higher than that in revenue for it to be worth the choice over a 5% rev share.
To be fair Unity is asking you to give them somewhere between 0.05% and 2.5% while Steam/Apple/etc. want 30%.
> AAA studio builds their own client and game engine, it's cheaper in the long run
I don't think it's because of the rev share. If they were able to acquire an engine which fully suits their needs for only 5% that'd would be a great deal. Unfortunately adapting an off the shelf for a AAA game might be just as expensive as building (or at least upgrading) your own engine.
Everyone takes their cut.