But why can’t they set their own selling and buying deadline
You can buy and sell at any point, regardless of the deadline.
It's registering the players that has a deadline - Uefa allows registration upto 31st August (or the next working day), the PL has decided that all players need to be registered by 5pm on the day before the first game. They can't stop clubs elsewhere in europe registering their players after the PL registration deadline.