Why do they make roads that cause porpoising?

I-5 north out of Bakersfield is so darn bad my Right-rear tie-down broke loose, heavy camper seemed to be bouncing about like a rag doll, I doubt this is too good on the superstructure... Stopped and re-centered camper and reattached fast-gun...Since then, I've added two more leaf$ per each side of the dually's rear. Empty, truck bed now 2" higher than stock, spring shop says that rear 'spring capacity' now exceeds that of a 4500.

Happy Camping,

3 tons