Don't quote me on this, but I've hurd/read somewhere that Valve employees can pick whatever projects they want to work on, but once they picked it, there's a "cooldown" on which they need to stick to the team before they can move on to other projects. They can indeed switch, but it makes it so that there's a sense of core within the team, and not people coming and going as they please. Because of that, or on top of that, given how little exposure and weight working on a project like TF2 brings to your curriculum - overshadowed by CS:GO, Dota and other IP's - most employees don't pick it. There's probably a Valve News Network video explaining their M.O.
I was reading about types of leaderships and company environments and came across this: "Laissez-faire leadership, also known as delegative leadership, is a type of leadership style in which leaders are hands-off and allow group members to make the decisions. Researchers have found that this is generally the leadership style that leads to the lowest productivity among group members." Ringing any bells?
P.S.: Funny how I'm two days late to post on this thread and it's already "old" because Valve released another update. What in the world...