Because then you'd have to have them having dialogue and speaking English and other languages, or subtitles. The humans are in these stories to give viewers something to follow along, it's a plot device, what makes the recent entries into the Alien and Predator fanchises weak is that the people are treated as just that and aren't developed into anyone you care about. Ripley and the marines were written so you'd care about what happens to them, Dutch and his men were done so you'd care about them. What weakens the sequels is that from A3 and P2 through to AVPR, the humans are presented less and less like someone worth caring about and the less you give a damn about the people who occupy 80% of the screen time, the worse the movie's going to feel. You watch the AVP's and you don't root for anyone, you just wait for everyone to get killed and see more action, but the action and death scenes are such small elements to the overall film.
a movie with only predators,would be interesting but has been done in the comics.
and as we have seen rarely do they make a good comic adaption or they start out good and end up stupid,like spider-man.but thats another forum Im sure.