Because in a lot of cases it's only the wealthy who can afford to send their kids there. Top end Eton and Harrow cost £50k a year and locally Whitgift or Trinity are about half of that.

Add in all the extras like uniform sports kit exotic trips etc it can be expensive.

But as I said earlier my issue is with the parts of this sector that are a commercial business. Where someone takes the profits. You run a nursery (and god knows they are bloody expensive) and you pay VAT. Run a Prep school and hey presto you are a charity! Also another perk is that if you 'donate,' to your private school (charity,) you do it under gift aid and so that's an extra little tax perk.