Because that's all they are. Uber and its competitors provided a valuable service in demonstrating what a modern taxi service ought to look like (at least from a customer perspective). But now that they've raised that bar, they don't provide a lot of added value other than as a worldwide integrated brand (in the case of Uber specifically).

Whenever door-to-door self-driving eventually arrives, it won't be in a timeframe that's remotely interesting to current investors in these services.