Experienced teachers are expensive, so why would you want to keep them, when you can have more inexperienced ones- until, of course, they become experienced themselves and so on and so on.
Successive governments and inspection regimes have ensured that teaching is a short term career- unless, of course, you are an academy chain CEO or have climbed the greasy pole in some other fashion.
There is a thread on AIBU which has attracted so much vitriol that it saddens me. Why would anyone willingly put themselves through the type of opprobrium so common these days, forfeit any home life or time for leisure pursuits and make themselves unwell because of it.