Because they're still film productions. They cost money (and if the best you're asking for is Dead Space quality I'd just as soon have nothing).
DC had animated series for a long time before they had their animated films. They knew there was an audience, and some of the earlier films were just compilations of several episodes strung together. Fox doesn't have that guarantee that there's a market in the AvP space for this kind of stuff so for them it's just as much a risk as a live action film.