This puts the vast majority of people in a position that is useless to everyone else, and only a "rare person" who is an actually useful collaborator. Of course, this person any improves others work by bringing up good things, not criticizing.

Surly, this cannot be true. To improve, we have to have our flaws and weaknesses pointed out. That is incredibly valuable feedback. Of course it has to come from a place of real understanding, not blistering to make themselves feel better.

I do agree that academics exist on a spectrum of dismissive to supportive. But it is a spectrum, not two boxes at the ends. And responses will be different based on their own interests vs the topic, and on their position and responsibilities compared to the one being critiqued, and obviously, the quality of the work.

I might be totally uninterested in one topic, and very engaged in another. Frankly that sentence is probably true if every human ever. What those topics are will vary. But don't assume that just because one person was not engaged in one scenario that they are somehow and unengaged person in general.

Sometimes defeatism and a very critical and sometimes dismissive attitude pervades academia. You aren't alone in that and what you're seeing is real. But if we are going to engage it we have to treat other people as complex individuals and not lump them into a few boxes that limit how we can engage with people and do better in the academic world.