Because you don't want to be pre-empted.
This means "bidding" more won't protect your capacity, AWS will just take it if they need it.
Prices are now relatively flat, and changing slightly over time by a fraction of a cent per day with a cap at the OnDemand price, while previously there used to be lots of from 0.1x to 10x the on demand price and with swings even multiple times within a day.