Because the alternative hypothesis to racist teachers is literally unspeakable.

… centuries of disadvantage compounding over generations? The predictable outcomes of poverty?

People talk about those all the time.

What? That Negros are dumber than Whites? I'm sure this has been debunked multiple times, so people generally don't say it for fear of sounding stupid, not of enraging some higher up cabal of leftists that either secretly or openly control everything.