Markdown was created in an era before the web had easily used components for structural syntax highlighting (tree-sitter) and where reliance on regex-based approaches was more common.

Using different delimiter for opening and closing is a good idea on its own, too. I think it makes parsing simpler and unambiguous wrt nesting.

I've imagined something like this:

  `(monospace)
  _(underline)
  /(italics)
  ~(overstrike)

O, c'mon! That is clearly a giant heading in a display font, squeezed in the middle and wider at the top and bottom. (-;