But they have to consider the folks coming in too. If you don't leave until 2 pm, then others can't come in until after 3 pm. They may want to be able to setup camp and have a coup!e hours to relax/explore before having to start prepping dinner. They may want daytime hours for their kids to burn off energy that first day.
And some campgrounds want/need more time between the check out and check in times. It will reduce opposing traffic on the narrow roads, it lets the camphost do a bathroom clean, reservation checks, etc., without trying to deal with incoming traffic at the same time.
Most places have check outs between 11 am and 1 pm. If you don't like the 11 am, then choose the places with 1 pm. If that's still too ear!y then pay for another day.
The end result is that if they all change their check out times to suit your preferences, then someone else will be unhappy. The current times have been standard for the past 50 years I've been camping and there hasn't been an uprising so I suspect it works well for the majority of people.