When you're in the ideation stage of a startup it's hugely valuable to be in the same room arguing over the same whiteboard. There's no higher bandwidth way of communicating, learning and building consensus. Once you've decided what to do remote can totally work, but most startups are in that first phase for quite a while.

There are lots of other reasons for being in the bay area in particular, but that's the primary reason for founders being in the same physical location.