Because for every HEMS pilot in the UK, there are 50 other pilots who would do their job for less money. And most HEMS pilots are ex-mil and hence, have an ex-mil pension and didn't pay a penny for their training (ie zero training debts).
But they are generally very well trained, got to be pilots because they passed a stringent selection and testing process, have served their country in some pretty unpleasant places and usually don't have anything like the amount of pension you think they do.