The classic paper format is just ergonomically what many of us are good at handling effectively as readers. For example in ML typically they all have an abstract, a teaser figure with a caption, Fig. 2 with a method overview/architecture (boxes and arrows). An intro starting with the motivation and the problem with prior work, their key idea, their experimental evidence, then a dense restatement of the contributions as bullet points. Then related work overview, then the method description in detail, then the experiments, dataset descriptions, protocols, metrics, then the results and their interpretations, then the conclusion, i.e. what they conclude from the results.

Its fairly rigid and newcomers often complain that it's too repetitive but if you read such papers for years, you learn to very quickly navigate such a paper that adheres to these conventions and you quickly see if it's something you care about right now or not. Blog posts don't have the same formal structure and it makes the quick skimming and assessment much harder.

> Then cite them as blog posts

My point is it's still useful to have a somewhat authoritative place to cite (high quality) blog post level content. arXiv has formatting requirements and doesn't go down like random personal sites.

> a LaTeX PDF can launder epistemic status

True to a certain extent, although something people are aware of and they can judge the content themselves (hopefully).

If you judge things based on their formatting, that's on you

At least in economics it can easily be 1-5 years until you go from draft to journal. In the meantime, you want a way for others to easily cite your paper, to make different revisions available, for you to post it in a way that's stable (people's websites change all the time, etc.)

Also, because most folks don't want to deal with paywalls, it's standard practice to put the last version of your draft before conditional acceptance on an online repository. It used to be SSRN for econ/finance, but they sold out to Elsevier, so now arxiv is increasingly being used.