I don't know about other festivals, but around here shops tend to have special festival offers, £30ish will get you a cheap pop up tent, roll mat and sleeping bag.
Less than the price of a night in a B&B, most of the time the tents are shit and not fit for use more than once, festival goers can't be arsed to take them down or cart them home so they just abandon them.
I've been up there with a local charity to collect tents post festival, it's not pretty - some are used as toilets, they get slashed, buried in mud, etc. it's horrible
The clean up teams go in with tractors and stuff, flatten the lot, scoop it all up and it's taken away. Job done.
People shouldn't drop litter, or leave their tents, but they do. The organisers encourage people to take their tents, throw away their rubbish, but you can lead a horse to water but you can't make it drink. It's not always the younger generation either