Yeah, and there was this guy who thought the world only had a need for 4 computers...
There will be plenty. For one, they're absolutely huge for the transport industry -- a driver that doesn't get tired, doesn't complain, earns no money, and isn't subject to labor regulations. The first company using self driving trucks will gain a large benefit, because trucks already go as fast as they can, so the company that has trucks that never stop will obviously deliver merchandise faster.
They're a huge thing for companies like Uber, too. If you think of it, a taxi driver is performing a largely mechanical task anyway.
Then there's the enormous personal convenience, a car that can drive you to work while you sleep, read, or finish some paperwork inside, then drives back home, drives your wife to the store, then drives back to work to pick you up. That, right there, is a car that's doing the work of two.
There's really no reason for them not to be ubiquitous. There's no physical law that forbids them from existing and it's just a matter of tech development to get there. There are multiple parties all working on it, precisely because it's so huge.