As noted, the bulk of the 'chute material degrades at a decent pace.
One question is: What stays up in the trees where it’s more easily seen?
There could easily be 200lb+ of weight pulling on the cords. Given some wind, snow, etc. pulling on the degrading fabric it wouldn’t be long and it’s on the ground. Then it gets covered in forest litter. Finding it once it’s on the ground is really difficult.
That assumes the parachute was opened. It’s widely believed that Cooper could not have been able to stabilize himself and correctly pull the chute open. I.e., it was all freefall straight to the ground.
Compare with the Steve Fossett case. Disappeared in a light aircraft. Extensive search in a less dense forest (finding other wrecks along the way). Finally located a year later when a hiker found his wallet. Only two bones were found: a half mile from the wreck.
And this was a (small) plane.
People who don’t know wilderness vastly underestimate how hard it is to locate something like this when you are deliberately searching.