Fenders need to be adjusted and tightened down. They can interfere with rack use - they certainly made putting on low riders more time consuming. They limit tire size options. They can get rub when the tire isn't true or when they creep. They add weight. They decrease space between wheel and dt so that you can't get he wheel over some racks. They can collect snow and mud.

It's reasonable to decide the negatives don't outweigh the positives, but there ARE negatives.

Personally, I'd rather use rain pants and a ti frame. I lie fenders on some bikes, but it's more aesthetics than any actual advantage.