Ah, so cynical, Cmdr. Bello...
Its a valid point, although I'll offer two counterarguments:
(1) No matter how extensive an editor is, an expansion pack can always offer new features that weren't possible before. For example, even if vanilla Civ 3 had an editor that let you add in as much custom terrain as you wanted, there would have been no way to add in volcanoes until Conquests came along, because volcanoes behave differently, and there was new coding necessary for that. Granted, there aren't terribly many exampless of such things, but the point remains: expansion packs can change the fundamental rules of the game, while mods can only alter parameters or add new elements within the existing rules (no matter how good the editor is).
(2) I feel you're overestimating the quality of most modders work. I've looked through the graphics created and posted at this website, and while some of it is as good as what the professionals include with the game, much of it is not, and I would say I haven't seen anything significantly better. This is not a slam on those who make such units/splash screens/improvements/whatever: I realise that its a lot of work, and most of what's been done is quite impressive for amatuers. But I, for one, would still be willing to pay for a collection of professionally done units/etc., especially if they had statistics and abilities that had been playtested. Sure, a few talented fans could produce some professional-quality artwork to compete with an expansion, and some enjoyable mods would get made as well, but I think "within days" is an big exaggeration: thouroughly playtesting a mod to make sure everything you've added or changed works with the rest to make a balanced whole is not a trivial task. I've found that the mods I'm most interested in are the ones that offer only small tweaks to the default game, since I know the default game is pretty well balanced, but you never know what you get with somebody's mod. Its like a lot of things in life: you get what you pay for.