Except devs aren't, and shouldn't, be developing under that assumption, they should be developing under the assumption their game will be successful. You don't want to be giving your pitch to investors and have to go "we aren't using AWS services because when we fail we'll provide the exes to the users".
And if you think they need to change, your just admitting this will cost devs more (and when it costs devs more, it raises the barrier of entry, in an industry where failure is already the norm).
The most obvious example is pretty much any form of inviting a player/having idenities. The storage of users and inviting them is what brings in the scaling complexities in your average online game, and that's when you'd use a service harder to have a self hosting equivalent of.