There are a few different scenarios but outsourcing firms / consulting (infosys, tcs, wipro etc) take up about 1/2 the tech h1bs.

As a body shop you can charge a higher rate and get bigger margins on an on-shore body.

I see your point about faangs and direct hires though. I suppose they must believe that something about being in the U.S. makes those people more productive or their output more valuable.