But why who blocks them? I may be biased because I'm from a country where PoC are < 1%, but all other people have different skin colors, some are very white, some are very dark (probably descendants from old Tatars).

So I'm genuinely curious why PoC might get problems with getting interviews? Do US based employees biased in some way, still in 2021?

All that matters for me in interview if is that person smart enough and nice enough to work with.

I don't see a lot of black people in IT, but I see plenty of Indians and Asians - those are also PoC I assume.