Why is the oil pan so hard?

The idea is to build the engine so you don't have to pull the pan in it's lifetime. Usually if there is something wrong in the bottom end, you'll need to pull the engine anyway. With the engine out and on a stand, repairs are so much easier. Not worth doing them in the chassis with oil dripping down your arm and/or into your face and hair, then finding out you need to pull the engine anyway.