No, they wouldn't have already done so, because their current job still exists and it's easier to maintain the status quo.

Major structural employment changes don't happen overnight. This will take decades. AVs will decrease the demand for Uber drivers, but it doesn't mean they will all be out of a job overnight, just that incentives will slowly push workers towards other jobs.

Uber and Lyft kind of did shutdown the taxi cartels across the world in one big grassroots swing.

It took insane levels of lobbying and political power of taxis to keep a hold on some of their biggest cities, and there are now few places where non-uber/lyft style systems are still in place.

If AI taxi's are cheaper and more convenient, there's no way they won't just completely replace human taxi drivers. Cruise and Waymo have huge incentives to undercut whatever Uber would try to do. So I don't share your optimism that it will happen slowly over decades. If this self-driving system is easily repeatable elsewhere, it will likely be fast and swift.

It's easier to maintain the status quo, but there is also job enjoyment.

I am neither a deliverator nor a cab driver, but I do enjoy driving. When the org I work for needs stuff transported, I often volunteer for the task. Driving in silence, or to music, or while listening to podcast; for work, for recreation, or merely to enjoy the drive; it's all good. Of course, I also live in an area close to mountains and while going toward the city nearby is hell due to traffic, going toward the mountains is blissful and beautiful.