It is happening today. We've always had some vanpool-ish type companies, though they haven't always been easy to work with. There are some startups trying to change that, like https://www.chariot.com/ -- I've been keeping an eye on them and when they start operating along my commute line I will likely switch away from BART.

The big problem with vanpools is scheduling. Everyone wants flexibility and smarter scheduling systems in a manner very similar to what uber has provided for individual rides.