The oroblem with collagen supplements is that for the body to absorb them they have to be broken down into bits - the same as any other protein. Rather like dismantling a lego model to push it through a hole.
Just as you can make a multitude of things with lego, the body will use these bits to make whatever proteins it needs.
Any balanced diet with plenty of protein is just as good at supplying these bits, and there's twelve time as much protein in 100g of baked beans as a typical collagen capsule.
Usually the collagen in the capsule is already broken down but the gut is damn good at digesting protein anyway.
The other issue is the body is full of collagen and won't necessarily target the joints that have wear.