There's little benefit to it as an input system on iOS/iPadOS (likely the dominant platforms for Notes) where formatting menus are just as close as `#` and `_` characters.
Several Markdown rules wouldn't make sense in the context of Notes. e.g. "end a line with two or more spaces then press return to create a <br>", which was designed to accommodate manually hard-wrapped text that Notes users likely don't want. Apple would have to follow something like CommonMark (feels unlikely) or implement their own Apple-flavoured Markdown, leaving you to learn what's supported and discover the quirks — kind of like its partial implementation of vim input in Xcode.
Popular Markdown apps seem to have converged on 'edit on line focus, preview on line blur', which is surely what the Notes app would do, because modal editing with preview and edit modes feels un-Appleish. 'Preview on line blur' _is_ nicer than a separate preview mode if you're a Markdown power user, but it still leaves many quirks you have to learn. (Just today I wrote, '# Thoughts on C#' in Obsidian, which reads ok with the cursor on that line until I pressed enter and the preview became, 'Thoughts on C'. Leaving me to learn I was supposed to know to write '#Thoughts on C\#' in the edit mode.)