The last two posters hit it on the head. Games aren't about engines any more - that's why they get licensed. Engines are... well, not easy, but easily reusable.
It's all about the content: textures, story, modelling, scripting and whatever. THAT takes all the development time.
It's like film: anyone now (with a decent digital video camera) can make a film, but unless you have ideas and talent and a script, you aren't going to be bothering Hollywood any time soon.
It's so sad, but also so interesting, to see a mod team saying "We're going to do this and this and this and make a whole new game." More often than not, as soon as the scale of the task becomes apparent, the mod team gives up, slowly. And those that don't go on to work in the industry, which makes a lot of sense.
Uh... ramble over.