How do you know they never consider it? I've not heard anything specific from NASA, but they do seem to have plenty of people who do dream up long terms plans and ideas. Don't forget that most of the people there read/watched science fiction just like we did and many of them were inspired to take up their careers at NASA because of it (see various bios on the NASA sire if you don't believe me).
The problem comes in turning those blue-sky ideas into reality. There is no 'just' when it comes to space. Whenever you find yourself asking, "Why can't they just..." it is almost always for a good reason. A mothership's a great idea, but how do you build one. You've got to get the parts into space. Are you going to test the nuclear propulsion? How long will this project last and will Congress give you the funds? These are the sort of real questions that need to be answered and have scuppered programmes before now (and look likely to scupper NASA's current plans - which are less ambitious than the mothership idea).
The way I look at things, I ask are they likely to cost a lot more than what is happening now. To do this, you need to assume there will be no miracle leap in technology in the short-medium term. For example, if raising parts to space cheaply relies on a space elevator, then rule out the short-medium term. Obviously, we have ideas that one could be built, but technological breakthroughs need to happen to make it a reality. While this is possible (and even likely, I hope), don't assume that we will have a functioning elevator within 20 years.
So, using only slight advances in current tech, could we build a mothership for approximately the same as what the ISS cost? I don't believe so - it would clearly need to be bigger and would involve research and testing in propulsion systems. Given that the US, Russia, ESA, Canada and Japan are struggling to find the cash to keep the ISS up beyond 2015 (when most of it is already built) who is going to fund and build the mothership?
Sorry to sound so negative, but I get fed up with all the unrealistic ideas and whining about NASA on Slashdot. Being a Brit with a government that does no funding of a manned space programme at all, I think they do a fantastic job given the resources they have to work with.