Planned obsolescence.
I saw a neighbor boy’s girlfriend smearing toothpaste on the boy’s car headlight lenses one day. I asked her if she was playing some kind of practical joke. She said no, just making the lenses clear again.
I tried it myself on my own car, except with automotive rubbing compound. Although it took a little elbow grease, I had clear lenses again. I decided the kid had a keeper of a girlfriend.