Why do car boot sales start so early?

My mum does car boots sometimes, she recently moved in with someone who has a considerable amount of random knick knacks in his home, like antiques and whatnot. But yeah, they say the same thing - they have to be there at 6am to get everything set up. It's mind bogglingly early! I went with her once, it was actually pretty fun. We probably made around £160 that day, just relaxing by the car, eating sandwiches and drinking flasks of tea/coffee. We were keen to get rid of things so if someone wanted to haggle, I didn't mind. I got to wander around the other stalls as well and see if there was anything interesting. We were away before 12pm so at least we had the rest of the day. Definitely more beneficial in summer since it's still reasonably cool outside when you arrive. By 12pm everyone will have definitely had enough as it gets hotter and hotter.