Bless your sweet innocence Backpacker. If only gliders only cost the price of tyre wear to operate!
Depreciation? You've got to buy the glider, or make provision for an eventual replacement
Insurance? Especially on a club training two seater - several thousand pounds a year.
Paperwork for the annual inspection even if volunteers do the work.
Wearing parts - release hooks etc
Minor repair materials
Periodic fabric replacement if it has fabric
Then there are all the costs of running a gliding club - airfield rental, maintenance (grass cutting), hangar & building rental or upkeep. Electricity, water, taxes.
Launch equipment and ground vehicle repair and operation
They all have to be paid for.
Normally by a combination of annual subscriptions, flying fees (glider rental) and launch fees (plus bar profits!). Private glider & trailer parking charges.
It's just up to the club how they divide up those charges. If they don't charge a rate per minute on the club gliders, then the other costs have to be higher.
The money has to come from somewhere!!