[jackowayed]

The reason is that the Bay Area is far more of a unit than originally, so things grew up independently basically in each county.

San Francisco, the city, has one main municipal system, Muni. Caltrain grew out of commuter rail that's existed for about a century and was originally private I believe.

BART was put together as an East-Bay-centric commuter rail that hoped to kind of unify everyone, but Marin (across the golden gate from SF), Santa Clara County (Silicon Valley/San Jose), and San Mateo County (between Santa Clara and SF) didn't want to pay the taxes required to be part of it.

And then there are other county bus-heavy transit agencies like Samtrans, VTA, AC Transit; it wouldn't make sense for Muni to run these since they're meant to serve other counties.

It's a giant mess, but each one largely sprung up on its own back when it was much less normal for someone to regularly travel all the way between Mountain View and SF.