The output of Markdown processing is not.
Markdown is intended to be readable without processing as its syntax is mostly based on plain text email conventions. You should consider the ability to process Markdown to another output format to be an almost incidental benefit to using Markdown, rather than its primary function; meaning, Markdown syntax conveys semantics even without processing.
Its line behavior is based on plain text email client conventions, and plain text editor behavior, which many do not by default perform line wrapping, nor necessarily should, depending on user preference. Meaning, a Markdown file should be readable with line wrapping turned off, hence why it consumes line breaks, unless you use two spaces at the end of a line to override this behavior. (Think of the two spaces as an invisible rendering hint to Markdown processors.)
Email line length specifications: https://tools.ietf.org/html/rfc2822#section-2.1.1
> and plain text editor behavior, which many do not by default perform line wrapping
Not true, in a plain text editor you can make ascii art, or a dashed list of items, without having to type special syntax, and it will not become one long line
Dynamic line wrapping for display is something different than removing newline characters ("\n") you type in the string. Markdown does the latter. Text editors do not remove newlines you type.
> plain text email client conventions
I don't think plain text email ever removes a deliberate newline you type either. Does it?