Earlier quoted context omitted.
Of course it does, if a programmer bothers to take the time to add it. Slack is a glorified instant message service, and instant message clients have been handling WYSIWYG for a couple of decades now.
So you think it's fine for me to type out a nicely formatted wall of text on a desktop that mobile users have to scroll to read?
This kind of thing can also be annoying in sites like Hacker News--e.g. when I try to use an actual asterisk a couple of times and the damn thing italicizes a big random chunk of my post instead--but HN at least has the excuse of a consciously minimal interface. Slack coats itself in slick, modern UI but makes you type emphasis like it was Usenet. It shoves giant obtrusive thumbnails into the chat every time someone posts a URL, but doesn't let you make an actual hyperlink. It's minimal where it should be complex, and complex where it should be minimal.