So, why do so many pay 40% income tax?
What do you mean by "so many"?
Remember that this board is not representative of the UK population, as
1) you have to be self-supporting in order to emigrate
2) there are people on HSMP visas (and their spouses/partners), meaning that they are in well-paying professions
3) there are people who have been transferred from jobs in the US(and their spouses/partners), meaning that they are considered indispensable, and are therefore well-paid
You could just as easily ask why there are so many people in the UK who are on benefits. (Even though there aren't very many people in that category on this board.)
The average full-time salary in the UK is about £23,000 a year, by the way.
Perhaps the reason you hear about 40% tax so often is that the ones who pay 40% tax are the ones who complain about taxes most?