Because its your club. Yor membership fee goes to pay the rent, the insurance, maintainance etc etc etc. If you started a flying club from scratch, you'd need a mega-load. Where would youy get the money to repay it - from the people who use the facilities.. so you either add £10 to the cost of a lesson, or charge £110 p.a. membership.
There is no such thing as a free lunch.