I suppose it is like comparing McDonalds to Michelin starred restaurants. There are, as you say, plenty of rubbish schools out there who will employ anyone. But good teachers who learn about their craft and care about their work will be able to find work in better schools even if they start off in poor schools.

I have been fortunate enough to meet many great teachers. I have also met many teachers who don't care and dislike teaching. Naturally this latter group will end up in unsatisfying jobs in poor schools; but the good teachers have stayed in the industry and many work for places like the British Council or universities around the world where the work they do and the passion they feel is rewarded.