Headphones - plastic foam cover

I have a couple pairs of expensive headphones; AKG and Bose (I believe). On both of them the plastic covering the foam for the closed-ear cups has just completely disintegrated and fallen off? Why would it do that and what can you do about it?

Age, wear & tear, environmental factors (solvents, high/low humidity), etc.

Buy new ones.

On my AD-500x, the earpads have a thin plastic of sorts on part of them that is flaking off on the left pad but is just fine on the right pad.

Can you buy just new foam pads with the covering? I guess I have to go to the maker’s web sites and see if they have parts available.