This is the part I don't get. Right now they rely on their employees to buy, maintain, and depreciate their own cars. The natural next step would be to let people lend their cars to Uber/Lyft when not in use.

But they're sinking tons of money into self-driving, which means they would bear the costs of buying and maintaining their fleet. That's a completely different business model, and seems like a tough pivot. And as mentioned elsewhere in this thread, other companies are better-positioned to do this, with better manufacturing, more sensor data, more experience, or all of the above.