Why do stock leafs sag?

I know that being designed to lay flat under normal load isn't helping, as well as carrying too much payload. I also know that short add-a-leafs and lift blocks don't help either. What else contributes to the stock leafs sagging?

Do extended shackles cause the stock leafs to sag? I'm thinking of using some 1" shackles to raise the rear a little. Will I have to do anything else when adding these, like use degree shims on the axle?

This is probably one of the fastest ways to get there:
4.0L + SOA + soft springs = springwrap = saggy springs
But overloading them or age can lead to sag on their own. The lift blocks contribute to greater leverage placed on the springs, increasing the springwrap and the sagging happens even faster.
Jeep on!
--Pete