No forum, email or word processor (even WordPerfect for the c64) or Notepad uses this
I think the convention of 2 newlines for each paragraph is a longstanding norm in plaintext. The old Usenet, list servs, plain text email, etc., was basically always like that, because you could never control how someone else wraps their text. 2 new lines would be a new paragraph no matter what, while single new lines could create ambiguity between an author’s intentional line break versus the rendering software’s decision to wrap an existing line.
For lists and the like, you’d want to be able to have newlines without new paragraphs, but you’d generally want ordered lists or unordered lists at that point.
For an obvious example of markup languages where newlines and carriage returns don’t have syntactic meaning, look at literally the most popular one: HTML.
So markdown was essentially enforcing the then existing best practices for pure plain text communication, to never use single line breaks except in lists.
Most UIs don’t even have a preview option, let alone need one, because they don’t require you to have a stick up your ass to ‘get’ using them.
It was pretty common before Markdown took over that forums and other user-input rich text fields used raw html (or a subset of html tags), or something syntactically similar to html’s opening and closing tags (BBcode, vBulletin markup, etc.).
Markdown was basically the first implementation that was designed to be human readable in plaintext but easily rendered into rich text (with an eye towards HTML). It’s not a coincidence that it took off in the early days of the “web 2.0” embrace of user-submitted content in asynchronous forms.
I get the complaint. But I think markdown makes a lot of sense as a way to store and render text, and that one compromise is worth it overall.
I’d go even further than that, and point out that the reason why computers sort things in this order is because that’s the most logical way to convey specific dates.
Most significant digits on the left, descending left to right, in order, is how we do all other numerical representations. It’s only dates that we have different norms.