Check in / out times are set to fulfill travel habits of most travellers. There are the few early and late risers, few weekenders who would like to arrive early, or stay late.
In some places with an amount of weekenders I have seen they have policy to accommodate those campers, but at what cost or annoyance to the majority of others who deserve their quiet time. You want to arrive very early or stay late into a day, reserve the day before and after.
Arrive early, , leave late when you want. The camp has little to no chance of rescheduling your site.