So much gear, so little time

Have you ever noticed, when packing for a normal camping trip to a state park (normal camping trip meaning big tent, drive up campsite, water and electric), why do we pack so much?

Now I know that there are limits to how much one can carry in a backpack, so packing light is a necessity. On the other hand, why can't we pack the same way when going to a drive up campsite?

I have to say, I've become better at it, but still find myself with a packed and stacked vehicle. The only things that I bring that are different from backpacking would be a bigger tent, a large air mattress and pump, and regular food. Why does this tend to take up so much more room?