Problem is they do, or people wouldn't bother with them. I think the problem is nobody knows exactly how they work, although all the anecdotal (and some clinical) evidence suggests they do.
They work in humans too
Not quite sure which part of my post (assuming it's my post ) you're referring to, presumably the bit about joint supplements?
The clinical evidence does show that a good joint supplement will significantly maintain the mobility of the joint and slow degenerative wear. That in my books is a good enough reason to use one.
However, it cannot and will not reverse arthritic changes.