The original Doom and Wolfenstein 3D weren't designed for modding either, and look how many TCs there are for them (in case you don't know, tonnes)...
I agree with you about how hard it is to create entirely new areas, but a TC doesn't
have to use brand new ones. There are, after all, plenty of areas from BG1, BG2, IWD1, IWD2 and Planescape to make use of... plus you can easily modify them in IETME or even Paintshop Pro or Photoshop;). From what I've heard of upcoming games like Doom 3, future FPS editors are going to be even more complex and laborous than 3Dsmax, plus it sounds like it'll take something like 10 minutes just to make a single texture, what with all the extra layers you need to apply (bump mapping, etc).
Creating a new creature also takes a heck of a long time and effort in the IE, but the same is true of any sprite-based game. I cite Doom again as an example - it has TCs of Aliens, Batman, Ghostbusters, James Bond, an anti-terrorists TC, a western TC, and numerous others. Starcraft is another sprite based game that has tonnes of TCs (admittedly area design for it is a cinch).