Well either:
A) they are absolutely loaded and therefore can afford to hire a lot more than I can;
B) they are able to shell out the £6k or so and due to little or no debt can afford to instruct full time (although most PPL schools seem to be struggling for business and there is a large number of instructors looking for work at the moment; or
C) An operator gives you a break and a first rung on the ladder.
I'd appreciate any other thoughts you have.