Lots of Predator sequels and yet I have a Question?

The original Predator stared Schartzenager and he survived the story. P2 stared Danny Glover and he survived the story. Why haven't we seen either of them in a sequel?<div><br /></div><div>Schartzenager, at least seems to love doing sequels. </div><div><br /></div><div>Edit: I think I'd love to see a Schartzenager &amp; Danny Glover team up taking on the Predators.</div>