Dachande explains this perfectly. Humans are prey that shoot back, they require stealth to take down, it's not just a prowess contest (which most yautja usually excel in) so the reason Human's are so prized is that they can think they can reason. This is also mentioned in "Predator: If It Bleeds" when One Eye notes that Hunting humans is an "experienced hunter's game." In the story One Eye is leading a group of freshly blooded hunters (they killed their first xenomorph) and most of them get injured and all of them lose to the vikings. It's not that we are smarter, or faster, it's just that we can actually come up with plans, we can strategize and put them on the defensive, force them to be the hunted. That's not something xenos can do
