The point is that markdown could have todo's or virtually any other aspect that org mode supports put into it without changing the markdown standard and layer org functionality and various features on top for example todo's could simply be italic's starting with todo: so `# todo:todo hello` could be parsed as a todo entry marked as todo.
If you want to use markdown for Agenda and calendaring in Emacs you'd have to invent a different subset of markdown, most likely poorly supported anywhere else. And if you get there, then it begets a question - why? If we already have something far better and it's called Org-mode?
(Though Markdown's inability to nest Markdown inside HTML elements is a terrible default. HTML semantic elements containing marked up text is basically how I write documentation to myself, and obviously the right way to mix plain text-like markup with named, extensible annotations beyond the default syntax.)
I think most of the friction comes because you have to keep converting free text with bold, italics, headings, etc. between the two, costing time when you're moving notes, documentation, README.mds, code comments nowadays in Markdown, etc. back and forth between the two. That creates a dilemma: Should I write README.org or README.md in my personal project? Should I write Org notes about my work to help me track of WIP and notes that might become documentation, or Markdown blocks inside Org, or Markdown notes? What if I'm collaborating with others, or think I might in future? What about collaborating with LLMs?
It's just enough friction that I ended up avoiding Org for organising my work, even though I like the idea in principle and use Emacs all the time.