Presentations should have as little text as possible, but Markdown is a text-first markup language. I feel like if I used Markdown, I would end up putting way too much text on the screen because it's what's most convenient.
I guess, the way I view this is as a "better beamer" (which isn't saying much IMO) rather than a better powerpoint. Basically, it's a way for people who were going to make a text-heavy presentation anyway to produce something that looks OK, and to avoid using a heavyweight tool like LaTeX.
you could just as easily paste or type paragraphs worth of text per slide in GUI presentation tools.
It's more a problem of not thinking about the volume you're putting on a slide or not using a tool that gives you good visual feedback like a GUI app does as you add text.
I've been using Deckset https://www.deckset.com/ for years and tend to have very little text per screen.
There in also lies an issue with Markdown presentations, more complex layouts (e.g. a side by side comparison) often require tweaking css/html and are more inconsistent. Unfortunately I find it very hard to avoid layouts like these for deeply technical content.
History and momentum is part of it, but the other part is the value of the slides when viewed later. There's an inverse correlation between the usefulness of slides and the quality of the presentation!
Best I've seen is low-text Google slides with heavy speaker notes included, but that's more work and prep than most people are willing to do (myself included).