Are people afraid of being called out by annoying toxic jerks? This sounds like the only reason people wouldn't want to have their DPS made public.
But after being called out by one of those jerks, you could take the feedback positively regardless of the words that were said. Having yourself being brought into the spotlight for a moment to give you the realization that you are under-performing should be replied with "I need to find a way to get better!" not "I should ignore that jerk because he was being mean about it."

Yeah some people have no tact and cannot be polite regarding things. It happens. When you get down to the end result, you may not yet realize how much you have been under-performing. I have no idea about myself I don't parse. But I've been called out before and I've used it to improve. I won't dismiss advice helping me just because I was told I shouldn't be living or something to that effect.

If there's some other reason people don't want a public parse, I'm all ears.