> but to me it seems that you can jump in and out of employment with Uber & Lyft quite easily, to the point that it does seem more like contract work

You can't really look at one aspect of it and make a determination, it's based on several factors. The main ones that come up in this scenario are if the contractor/employee can set their own rates, and how closely management monitors how they do their job. Flexibility of employment - either in terms of scheduling or to take periods of time off - is not considered.