I've thought about this a fair bit since I gained my own interest in AI, and for me its answered thus: Because science, in general, is pretty glum about intelligence - in that there is no definition for it, and its not very well understood at all. To the point where any real headway made on the subject runs into the religion question; and we all know that sci-fi authors/writers are terrible at religion, in general.

Until there is a reliable definition of intelligence which addresses the substance that seems to be behind it, which science refuses to address (the soul), there won't be much progress from the sci-fi .. since the solution in the sci-fi world to the problems of all of sciences hole, is thus: there is more to us than intelligence. That's not something that a lot of folks can deal with, alas ..