2 reasons basically.

1. Because people vote with their wallets and not their mouths, and most companies would rather have a cost accident (quickly refunded by AWS) rather than everything going down on a saturday and not getting back up until finance can figure out their stuff.

2. Because realtime cost control is hard. It's just easier to fire off events, store them somewhere, and then aggregate at end-of-day (if that).

I strongly suspect that the way major clouds do billing is just not ready for answering the question of "how much did X spend over the last hour", and the people worried about this aren't the ones bringing the real revenue.