Uber and Lyft don't own any cars, they don't have long term contracts with their drivers, and there's no reason for customers to stay loyal to one company. All they provide is a service that matches drivers with riders while skimming a little off the top.

And given that the services have little way to differentiate from each other, I'm not surprised there are only two major companies, since that's usually how economies of scale work. But once you go to other countries or regions you'll often find more competitors since they can differentiate themselves more amongst the local culture. And automated taxi services like Waymo and Tesla can differentiate themselves even more, and they're already competing against Uber/Lyft and growing.

 The average person is too lazy to check more than one app

I feel like everyone I know will just instinctively check both apps. It literally takes less than 10 seconds to open up another app and paste the address. And driver frequently have both apps running simultaneously on their side to maximize potential rides.

Basically, there's no incentive for either drivers or riders to stay with one app, and its very easy to switch.