I think it’s the other way around. Hourly wages are paid directly for time at work

At least in the US, there are regulations around what constitutes "full time" employment, and many benefits (such as insurance) are tied into being offered only for full time employees, or at different tiers between part and full time.

As such, you are still expected to work a minimum amount of time. That's what you're signing up for. Fixed deliverable contracts- completing certain objectives- tend to either specify those things as minimum performance expectations, or are for contractors rather than employees.