Average festival attendance - Glastonbury 135,000, Isle of White 60,000, T in the Park 85,000, erm, Mannifest maybe 3,000(?) but my point is that where on earth are you going to put that many secure lockers on a small campsite?
It boils down to this: Caveat Emptor. If you take a big bulky expensive stereo or a Rolex watch to a festival, you can expect it to get pinched, soaked or covered in gloop. Lock your cash in the car and just carry what you need for the next couple of hours. Simples.