In general, I'd say that's because it's a position that's shit on.
You're apt to be paid far less than actual dev positions. If you're a QA manager you're always pushed on by upper management to outsource and lower costs. There is none of the prestige of being a "QA 10xer" that you'd see heaped upon a dev in the same position. And I see little training/courses pushed out for QA like is typically seen for dev.
It seems like QA in most companies is a necessary evil that management would take out back and shoot the first moment they could.