It has nothing to do with left or right, you yourself is saying the left-right dimension is antiquated but still using the term to try to pull your argument.

It has to do with people incapable of thinking in higher-level orders, to understand nuance; to understand systems and how they relate to oneself, others and societies; to understand more mature views of the world, and to be able to parse through those views. They are present in any part of the political spectrum but the more immature sides attract more immature people.

You could just have avoided the whole political jab, it'd be a more interesting conversation.

If you go to (excuse my outdated concepts of extremist communities) Parler to talk about LGBT in a positive manner you will get about the same amount of vitriol as if you went to r/FemaleDatingStrategy to call out misandry or if you went to a specific /pol/ thread to fight antisemitism.

I think the bottom line here is, vitriol is pretty much omnipresent among us. The difference between communities is how they run the ductwork to siphon it out of our daily conversations. That's where the discourse is formulated and fine-tuned to the specific needs of its members.

When you build bubble-like communities, you will get echo chambers that breed infantile subjects. If you allow people to call each other certain slurs, but not other ones, you will naturally optimize for resistate to the former. If you build a forum with usernames and perhaps even an upvote system, people will recognize and build up reputation bound to their names. If you make an Anonymous board, people won't care about reputation. These are just two options on a huge spectrum of possible alignments. Engagement-oriented platforms (by that I mostly mean social media and Reddit), are however a special case.

Maybe an anecdote makes more sense: A few years ago, in high school, I used to find joy in trolling. I felt especially at home on the imageboard that starts with four, but when the thread would scroll over the limit and plunge into oblivion I realized how little those three people that I made seethe actually mattered. To contrast that, on platforms that value engagement, it was and probably still is a lot easier to reply with something inane and watch the replies roll in. A single statement that would go into the archives mostly unnoticed instead made an impact on dozens if not hundreds of people. After getting out of my turbo edgy phase I realised that I hurt a pretty good amount of people in both cases, and it feels somewhat dishonest to believe that every single downvote, reply and slur hurled my way was born in infantilism.

What I'm trying to say is that when we increase the number of interactions, we as a byproduct also increase the number of "bad" interactions. When I talk to "bad" people with "bad" opinions, I try to recognize that even if they are 90% infantile garbage, the rest can come from honest pain and discontentment. But sometimes that's just being too charitable. People are hard.

As a lefty myself, I find that right/capitalist politics to remind me of the toddler stage when kids are claiming things to be their own ("mine!") and exaggerating all of their accomplishments (e.g. Trumpism). Lefties tend to have more of a notion of sharing and trying to be emphatic with others, but there's certainly a breed of self-righteous, angry lefties too (c.f. Linus Torvalds' angry outbursts).

Edit: Another thought - is the infantilism better associated with populism? i.e. to appeal to a large demographic, it's best to keep slogans simple (e.g. "Get Brexit Done" from the UK Tories) and having simple ideas that can appeal to people despite the ideas not actually being practical (e.g. "Eat the rich")

By that lens, it makes total sense that the more seriously one takes the right-left polarity, the more one needs a parental figure!