Markdown is a fileformat, while Notion is an interface which can utilize this fileformat. I don't see anything in notion which I could not fit in that fileformat and some folder-structure.
The problem is that making a good interface is a though and long task. It's not done with crapping something together which then barfs out some file. That's people just can't build it themself, because it would be months and years to reach a good quality.
The demand for local&selfhosted on the other side is clear: people don't trust the companies. Companies peek into your data, sell them to other, or just disappear one day. With something under your control this will not happen.
The problem is that making a good interface is a though and long task. It's not done with crapping something together which then barfs out some file. That's people just can't build it themself, because it would be months and years to reach a good quality.
The demand for local&selfhosted on the other side is clear: people don't trust the companies. Companies peek into your data, sell them to other, or just disappear one day. With something under your control this will not happen.