and why dont they have kits for the 4x4s? ford can get them for their 4x4s
It's the way they are designed.
The ford ifs trucks were designed in concept to support a raptor style vehicle (which, by the way, is meant as a desert truck), the gmt400's were meant to be comfortable pavement pounders with the occasional mucky job site thrown in.
You also have to remember you're talking something like 20 years in the difference between the original designs of the two.
Ford Twin traction beam 4x4 was the competing original design with the gmt400 and had more travel than gm could even dream of back then so they simply went after a different market segment (ie: pavement and work trucks).
For the most part, it comes down to the axles shafts, the control arm mounting points and the overall geometry.
Unfortunately, if you want a long travel gmt400 4x4 front end you're pretty much stuck with two options:
1. Mega buck and ultra custom one off suspension
2. Swap in a solid axle.
Sucks, but there it is.