Actually, there are fixed salary positions. From the answers til now it is mainly a thing found in US airlines and from there it spread across the globe. However over here in europe i have been flying mainly on fixed salary with overtime payment.
The difference is semantics though. And of course the safety of earning at least the fixed salary, even if you fly less than the included hours, which is the case during at least 4 months during the winter, with the current reduction in fleet size probably the whole next year. But from what i understand there are often guaranteed hours in the hourly payrate airlines as well, which amounts to the same thing.