The reason is because ultimately, scheduling shifts is going to be both easier to manage and more cost efficient than letting employees choose their own schedule.
Uber has a good idea of how many drivers they need at any given time. This ideal distribution of supply will never align perfectly with the supply pool's natural scheduling preferences. You can try to use economic incentives to force these two distributions to overlap, or you can just exert control (that you rightfully have over employees) and force your employees to work when you need them.
The thing is that Uber et al prefers if there is no schedule and law actually doesn't stand in the way of that.
How? As far as I understand, this is literally impossible with AB5, unless they get an exemption like all the other contractor-based industries.
I also doubt that the law forces companies to have a work schedule for their employees. From what I understand it just says that benefits are provided if employee works for more than certain amount of hours.
So to me it seems like it's all about exploiting their drivers (treating drivers like employees without providing advantages that employees have)