Was just thinking the same thing the other day. I guess it's probably because these trucks have become cheap enough that folks have the $$$ required for major mods. Hard to justify swapping a perfectly functional engine (or transmission for that matter) when you've still got a $300-400 car payment on it.
Think about what we've got going right now... V8 swap, 5-speed swap, body-integrated spare tire carrier, tube bumpers, etc. Pretty cool...