Because the difference in complexity is comparable to the difference between a sports car and a unicycle.
that.
mods for elder scrolls games regularly include near total rewrites, (if you ignore the ones that are just lake, 'aah, have a mace with 7000 damage from the start') or atleast modify a lot of the game files. if i remember correctly the Unreal tournament mods weren't that much more complex then the 'Forge' tools in Halo, (and game type creation.)
also the ability to install this type of mod is a massive security risk.
EDIT:
And can't you use the search function, there's probably been a topic like this most days for, err... how long has this forum been here?
http://www.moddb.com/mods/prometheus
Loads of very complex stuff. People coding in splitscreen turning the whole game to full intergalactic warfare games or 3rd person zombie. The editor allows for more than the construction set.
Where did you get that. some of these mods become their own game .
I guess those who said that Skyrim's mods are too complex compared to UT3 are creating assumptions without knowing anything about UT3 mods.
So I guess that UT3 will not be that different after all since you can create new weapons, levels, areas, etc.
Now I will be extremely disappointed if they will not add mods on consoles because it is very possible and it should also be possible to go through Sony.