Because it's not practical. There are few people in a position to do research on entire software teams.
Also, even when it's possible to research something, external validity (whether it applies in another situation) is often a matter of opinion anyway.
We still need ways to share what we've learned from experience. But I think case studies, telling stories about what we learned in particular situations, are about the best we can do.
Can Martin point to _any_ major successful project that he's influenced with his ideas?
If not, then that's clearly a astronomically enormous red flag. If his ideas are that good he should be shipping successful stuff left, right, and centre. I don't think he is.
I'd listen to someone who's shipped great things. I don't think he's shipped anything substantial at all, let alone anything great.
Also, even when it's possible to research something, external validity (whether it applies in another situation) is often a matter of opinion anyway.
We still need ways to share what we've learned from experience. But I think case studies, telling stories about what we learned in particular situations, are about the best we can do.