Why is it considered necessary to nail down the lid of a coffin?

my theory is: zombies do exist and we nail down the lid as a safety precaution. that's also why we bury them so deep. so if they get through the nails, there's 6 feet of dirt blocking the pathWhy is it considered necessary to nail down the lid of a coffin?
Zombies will always make their way to get brains, and there is nothing you can do to prevent this. Just be better at head shots as this is the only solution to Zombies.Why is it considered necessary to nail down the lid of a coffin?
I think we nail the lid shut so it doesn't pop open during the burial, and we bury them deep so one can't easily dig them back up and so we can't smell the stink.
To keep water and other things from getting inside the coffin. and they're burried so deep so that if it rains and floods the coffins won't rise up out of the ground