After thousands of uber tips, 100% of my drivers have said they enjoy the freedom and ability to work when and where they want without commitment. Employment destroys all of that and would mean many of them would lose this income source.
If people do want to drive full-time, that job already exists. There are numerous places to work as a commercial driver. In fact, Uber Black leverages exactly this and is routinely serviced by companies that employee full-time drivers.
The fact that many drivers don't choose to be full-time professional drivers is far more telling than the tiny but whiny minority that seem to not realize what rideshare driving is.