Actually - several of them are. For example:

"Another field study compared interruptions in paired, radically-collocated and traditional, cube-dwelling software development teams, and found that in the former interruptions were greater in number but shorter in duration and more on-task (Chong and Siino 2006)"

This is one of the things I find fascinating. You see the same kind of thing over thirty years as technology changes radically. This makes me think that there's something quite interesting happening in meat-space.

Startup environments are not average, and this must be why companies like Github and 37signals can be successful while having remote workers.

Just to repeat. I am not saying that you cannot be successful being distributed. There are lots of good and sane reasons to be distributed.

I'm saying that there is a bunch of evidence that being co-located is more effective.