system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes cassette tapes degrade in quality?
gpt
Cassette tapes are long plastic ribbons coated with magnetic material that has been charged just right with your recordings. A few things combine to degrade these over time.

To keep the long ribbons from tangling, they're spun into spools (thus the cassette). Keeping the magnetic material pressed together like that risks the charges from one spool affecting the others. The magnetic material isn't permanent, but just lasts a long time.

As you play the cassette a few bad things happen. 

First, the plastic can stretch. This will happen just by the way the tape is turned so the ribbon spools from one side to the other. Also, like the magnetic material, plastic doesn't last forever, just a long time.

There may be additional impact of playing the cassette as the tape makes contact with the cassette player. There are wheels to help move and keep the tape taught as it passes over the reader, and the reader itself. The reader is essentially a magnetic field detector, but the magnetic field is so small they need to physically touch.

While negligible each time, one thing that can cause degredation is the miniscule electromagnetic interference by both the electronics in the cassette player as you play the cassette, but also the effect of moving the magnetic material as the cassette spools the ribbon.

This happens for all kinds of magnetic media, including audio and video tapes, and except for the stretching bits, old floppy disks.