Because we need to get information down quickly, but still have the option of parsing to a presentable format. Markdown is not perfect, but it's far better for the efficient capture of information or document authoring. I recently built AS Notes for VS Code (https://www.asnotes.io). It's markdown based, with wikilinks, mermaid diagrams, with the ability to publish to github pages etc. There is no way I'm writing my day to day notes with HTML and anchor tags without it being a huge distraction. And HTML is so much harder to read in longform if you need to come back end edit your writing. Markdown is for humans where HTML was designed to be simple, but is ultimately for parsers.