The other point is that while an individual programmer might be more productive when he's alone in a room, a group might be more productive if they're all in the same room together. No point in having everyone write twice as much code per day if the code no longer fits together and nobody has any idea what everybody else is doing.
How much of a programmer's time is actually spent sitting down in full-on focus mode belting out lines of code?
Besides, groups of developers in large open source projects, distributed around the world, seem to manage fine.
People can only work in groups when you divvy the work up between them, or if the group all focusses on one thing (one guy holds the fencepole up, the other whacks it with a hammer. One guy codes, the other guy checks the code. Etc).
The little full-on focus mode I get is spent on working out which code I need to put where. The remainder, simple code-monkey work, can be done in a normally focussed manner. Constant integration with everyone else's work destroys all focus.