What may be a theoretical issue to you is a very real one to others. I have glossy finish upright piano, and over time the buildup on the exposed surfaces is difficult to remove. In contrast, the covered surfaces look as new as the day it was delivered 20 years ago.
You can certainly speak for yourself, but you shouldn’t present it as a blanket statement for everyone. If your memory serves you well, there are a couple of threads here discussing cleaning solutions for glossy-finish speakers.

