Yeah I don't do it for mainly selfish reasons but I agree that there are a lot of benefits to separating arguments into multiple comments in terms of improving readability and structure. Frankly, I commend you for doing it (and I'm particularly amenable to it because I like bullet-points). With that said, here are some reasons you shouldn't take too seriously for why I don't:
Selfish Reasons:
Practical Reasons:
And this is why I think people don't naturally do it this way. Lots of arguments have a "common body" of thought that it gets repetitive to include with each comment. Even when they don't, people tend to just not think of arguments as "graphs" of justifications. They think of them like a serial back and forth of people on a podium giving speeches and engaging in "rhetorical battle", and it's more fun and engaging to write them that way on the internet.
This is partly a recommendation on how to format multiple different independent points or replies in any discussion forum, but it's also a post I'm going to use as a link to explain why I'm leaving comments all over the place so that I don't seem as weird/feel self conscious and don't have to explain it each time.
Oftentimes, when someone disagrees or simply wants to comment on something you've said, they'll have multiple independent critiques of it. The standard way I see this handled rhetorically is to quote your relevant material once, and write a long post, mixing those critiques together. Other times, someone will make multiple different points or arguments in favor of a single underlying idea, or even write completely unrelated paragraphs contained in the same post, that are replied to all at once, in quote->response, quote->response, quote->response format. This makes arguments hard to track as layers of->response->counter-response->counter-counter-response start to pile up each iteration of the comment chain.
To remedy this, when I can, I like to split up these independent points into multiple comments. This makes it much easier to follow the chain of argument, and to see which objections or justifications stand on their own, instead of hiding behind each other. It's much easier to see when this is done that someone doesn't reply, or gives up a particular idea. Even when people are correct, often most or all of their individual reasons for believing what they do are flawed, and even when they're incorrect, often the objections others raise are unsound, too. Leaving each argument naked to individual attack makes it easier to see, both for the writer and the commenter, that a paragraph is either silly or the main rhetorical pillar behind an idea or rebuttal.