Don’t get me wrong, if you have a good jam or preserves it’s totally fine on a sandwich, and I wouldn’t turn my nose up at it. However, peanut butter doesn’t really need the help, and the extra sweetness from jelly is totally unnecessary. Toasted bread and peanut butter is all you need for a quality simple sandwich.
I’d agree that people always put too much jelly, I like a thin layer of jam/jelly opposed to globs of it, but peanut butter sandwich by itself is just too dry & chewy.