Why not just blood donors?

Obviously, because it wouldn't be much of a movie without needing the whole human being.

Aliens land, hit up the government for access to the blood banks. Government agrees, hands over the blood. Aliens depart. The end. Maybe 10 minutes if they stretched it.

That would have been to logical for this idiotic movie...

Look, the whole premise of an alien species needing our blood or DNA is preposterous. We human beings can't even share blood amongst ourselves - it has to be typed first before a transfusion. We certainly can't share blood with other species native to this planet. How on earth (no pun intended) would alien beings be able to use our blood?

And aside from that...any species so advanced as to use wormhole technology to travel would have to be able to synthesize whatever blood component they might need for themselves.

I cut sci-fi a LOT of slack, but this is one recurring theme that has to GO. No more hybrid human/alien babies, no more "we need your blood to save our alien race," etc. etc. Just STOP it!

Because as they needed blood, they also performed a test on each human that would kill them. The test was for a perfect match that did not need processing. That perfect specimen would survive the test. If they found the perfect specimen, they would no longer need any more humans.