People have kind of touched on this a bit, but I just want to say this as explicitly as possible, because I think this is a thing that's been kind of forgotten over time: Forcing employers to commit to their employees on some level was one of the most significant achievements of the labour movement.

Most people don't really want to have the 'flexibility' of working, because that also comes with having to find work on a regular basis. In some post-scarcity or at least post-austerity society this might be something people are willing to do, but most people aren't working because that work is something they have a passion for, they're doing it so they can do other things like have a home and a family.

Employers of this kind of labour would love nothing more than to schedule you whenever they felt like, have more part time employees at lower wages, and let you go if you miss even one shift.