My drivers fender liner on the cab side is worn. Like the tire is hitting it. I don't know if its an old thing or something current. What would make this happen?The pass/cab side is fine. Second pic is pass/cab side not worn.

 

It may look like oil contamination or similar of some sort of substance that’s grinding away at it. If I’m correct, the material to make it isn’t all that durable anyways. Isn’t it like a horse hair mix like they use for radiator shrouds or did they stop that? Regardless, it’s some sort of rubber and once a chemical sticks to it, it could further the process of corrosion.

I had a 90s 4Runner where the fender liner like yours started completely falling apart if even mud stuck to it.

 

Ok that makes sense. For years probably the master cylinder leaked above it. Brake fluid is highly corrosive.
Question answered.