This is a misunderstanding of why downvotes can bother people. I will try to put it in perspective with an analogy: Imagine if in RL, you were trying to talk casually with an acquaintance in public. Suddenly, you hear a "boo, [your name]." The boo explicitly uses your full name, not a shortened name, so for the sake of analogy, you know it was directed at you. However, you have no idea who said it or why. All you know is there was a "boo" targeted at you.

It is just negative noise, divorced from any grounding. Not unlike the psychology of a scary sound in a horror movie, it leaves your imagination to fill in the blanks as to why the noise happened. There can be many explanations for the noise, some of which will have nothing to do with you, personally. But the nature of it is still presented as if it is about you, targeted at you.

In my understanding, a criticism is usually considered to be something specific that you can engage with. For example, if you yelled at the acquaintance and they said "don't yell at me over nothing, we were just talking." That is a criticism and something you could take action based on. You could reflect on whether you were needlessly yelling and if you think you were, you could apologize and try to be more calm in the future. The noise, on the other hand, doesn't tell you anything clear. So, do you really need to know? What purpose is it serving?

The spirit of it I suspect comes in some part from the real life forum inspiration of internet forums, where the idea is you are discussing things publicly and openly in front of an audience, and so you might get cheers or jeers. But in practice, this is not really how the internet works. You don't know who is looking attentively at you and who is not even present. You don't know who is "cheering" or "jeering" at what point in what you said and because you don't have any chance of knowing who it is, you can only guess why. The amount of information a downvote gives is virtually nothing, despite what anyone wants to tell themself about adapting their posts because of it; and you can see this reflected in how people react sometimes when they get heavily downvoted. They can get defensive, but in a sort of flailing way, like they're trying to work out what the hell is going on. Because they haven't actually been told what the problem is supposed to be. All they have is noise, their imagination, and whatever coping mechanisms they have for dealing with the noise. In this sense, downvotes can be more like a cowardly (in that it is usually anonymous) taunt than actual information. This is not to claim the intent is always or even often a taunt - remember, the point here is you know basically nothing from it - but that the nature of its low information is easily experienced as more akin to a taunt, since there is nothing substantive you can do with it.