For the same reasons the music is crap, the movies are boring and inoffensive. This Slate article wonders just "Why is airplane music so universally bad?" and I think gives a good answer for both the music and the movies:
http://www.slate.com/id/2094629
It's just that I'd like to be surprised every now and then. But surprise is Public Enemy No. 1 so far as market research analysis goes. Which is why airline audio is the aesthetic equivalent to the prose in TV Guide or supermarket paperbacks. The notion of mainstream taste, when pegged to the bottom line, will always cant downward...
But look at broadcast network television (the overhead screens of the airwaves): it's sinking to a slow, deserved death, pulled apart by more and better consumer choices than ever. Just as cable, satellite, DVD, and the internet give nearly unlimited choices to anyone willing to pay a little, other options like laptops, personal DVD players, and individual seatback screens are ending the mediocre movie's monopoly on our attention.