Because you cannot deliver experimental physics breakthroughs or hardware with teams of two deciding every week what they feel like doing.

Have you ever thought about how much work it takes to deliver hardware? How many teams and processes and planning there are involved from physics level to software stack to deliver something usable? It's insane. And normal processor hardware is something people have experience with. For a physics experiment, virtually everything is custom made and you only have budget for a few fixes or trials on each component. There is just not much room for slack.