The point of markdown is that it is human-readable not only in "rendered" html form, but plain text too.

I think this explains the lack of viewers; they are simply not needed.

That doesn't explain it since that point is theoretical, in reality markdown is poorly readable even for the basics (table with a few bold red words in a cell easily breaks alignment / readability)

(also seeing all those marks isn't aesthetically good, hence the need for a viewer)