Web browsers are not document viewers. They are customer acquisition channels for massive tech ecosystems. They don't serve users, but corporations. AI features, VPNs, crypto wallets, and countless of other nonsense.
Markdown, on the other hand, is a public good. It would empower writers to publish their words independently. It would strip out countless CMS systems, frameworks, trackers, and ads. All those things where the money sits. Markdown is practically anti-platform. It's simple.
Why implement simple Markdown when there is already a bloated PDF viewer and you can play AAA games in a tab, or generate images in some chat window? Markdown is an alien in that world.
Not supporting Markdown in a web browser is a choice. That choice tells you exactly who the modern web is built for.