'Cos a giant purple space man would have worked so much better in a live action film? So things from the comics are just too silly for a film (even the more light hearted FF films), Galactus is one of them.
Fox have done a fantastic job on the X-Men films, even Origins:Wolverine and Last Stand weren't as bad as Hulk (2003) or Ghost Rider 2 (and I like that one).
As long as they keep FF running to the same styling's of the X-Men films they should be fine.
Also wasn't that female Doom thing a practical joke?