I don't know who "they" refers to, but the reason why people pull up their pants before they sit is because they want an unnattractive buttcrack showing it's face to all the people sitting behind you. That and most people would rather not be "saggin'" and have their underwear showing either.
So they don't like get stuck and get pulled down when you sit on them. Don't want anything showing to all of the people behind you.
What Girls Said 1
Well there is a very good reason for this I know many guys who wear their pants down so we all wear belts then when they bend or sit down the belt goes up and sticks to the back whilr the pant comes down as a result their ass is on show