I think a large part of it is wishful thinking. As a scifi fan, I think it'd be great to find out that we were aliens seeeded on Earth by some advanced civilisation. To weigh up the facts and realise that there's all the evolutionary evidence to show that we did just evolve from fish is a bit more boring.
Okay, maybe "boring" is the wrong word. But just think how cool it'd be if there were bits of alien technology lying around waiting to be found rather than just a non-rusting lamppost, lines in a desert and a few pyramids?
Interestingly enough, but as a teenager, I classified his stories as sci-fi. And I was absolutely into sci-fi(that was the genre I kept reading right after Fairy tales... yes, Ive read Bible in my early teens as well - nobody asked me to do so) - it made me learn English, as I have read all the ~100 sci-fi books in my language and there was nothing else to read for me.