It's Brian's stupid fault.

Earth Girls are Easy?
The Adventure of Buckaroo Banzai Across the 8th Dimension?

I think the main problem is that SF, and space opera in particular, can fall into self parody so easily if not done right that few bother to be intentionally over the top. You have to take the subject seriously as a director if you expect the audience to buy in at any level. The same is true of fantasy, of which there are even fewer successful films (pre-Lord of the Rings).

In anime, there are parodies and humorous SF shows galore, in part because you have a large body of serious stuff that you can riff off of. Even there you usually don't get full on parody (as you do in Project A-KO or Excel Saga) but instead a mixing of comedy and serious drama, either starting as one and morphing into the other like in Gunbuster or going sort of half and half like The Irresponsible Captain Tylor. One of the more amusing things in anime history is that the original Macross TV series (known here as the first part of Robotech) was intended to be a parody but people loved it as serious drama.