I'm curious as to why is squatting bad for your knees?

It can put. Undue stress on your knees particular when doing very deep squats loaded with heavy weights.
Its ok. Squatting itself is not a problem and is a normal activity, however if you do this repetitively, and it becomes painful, it could be causing damage. In some people, the meniscus of the knee can be injured with a deep squat. Again, if it doesn't hurt, don't worry.