"the system can just hijack the closest jeep instead"
You jest but perhaps that's the business plan - you call Ueber, they hijack the nearest car, up the aircon to unendurable till the driver bails out, then deliver it to you to take you where you want to go. The next thing will be that they sell anti-Ueber-hijack insurance.
On a more serious note, the idea that the Ueber drivers will do the work to provide the income to design them out of a job apparently has its parallel in the canals. Some of the canals in the UK reached the height of prosperity hauling the materials to build the railways. Without the canals, the railways would have been too expensive to build. Without the present road and fuel infrastructure, the opportunity would not exist to fund self-driving car development. And so it goes.