Basically, there are only the top guys. Not enough independent work to support many more than them.
Things were different in the '60s, '70s and even '80s when a lot of band's drummers weren't really studio-ready, and 'ringers' (ghost drummers) were brought in. But the overall skill of drummers has really escalated in the last few decades in particular, so they do the recording with their bands, and a fair bit of recordings now are simply done by a guitarist/keyboardist/producer who don't really need a live drummer. There's not much left for the independent studio drummers.
Bermuda