Take a look at the markdown spec. Unlike HTML markdown is entirely dependent on well-placed line breaks.
An errant line break in an <h1> element wouldn’t have any impact on rendering, but:
here in markdo
wn world it does
This is true for lots of elements, markdown tables are horribly broken by additional line breaks. HTML tables OTOH are not.
I’m not sure the HTML analogy sticks or is useful, users aren’t being asked to write HTML, can you imagine the pain of missing <p></p> and <br>? Literal walls or text.
If you think “markdown first” which Discourse is, rather than “HTML first” which Discourse absolutely is not, then the challenges presented by throwing tags in without respecting structure begin to make sense.