True, true...
Modding is quite tricky and it isn't the same as customising. If pigs would fly and R* would make a simple app for installing mods, it still wouldn't have a 3D modeling, coding, texturing, converting program included.
If you wanna make mods, you'll get the tools you need. If you just want to see more customising features (player model, haircut, clothing, modding cars at stores...) write that in GTA4 wishist. (although nobody reads that... )