Why are people breeding dogs with problems they can't get rid of?

I saw a video where the dog had a condition where his skin would get red and itchy and the owner showed an example of the medicine he needs. I don't understand why people breed dogs with a condition they can't get rid of, it causes them pain, makes them uncomfortable and makes their life worse. There's an endless number of dog breeds with no conditions, why does this exist? Why are people doing this? Also why would someone buy a dog with a condition like that?