One of the problems is that people want different things from a "simple document format".
Some want rendering-agnostic structure, like Wiki / Markdown / Org mode, that emphasizes structure and text, with things like links, tags, etc.
Other want to print centered headers on level 1, and left-aligned for level 2, and body text with a particular font, with fragments in a different color, and pictures with text flowing around them, etc.
These two camps command different design goals. A format that makes one of these easy adds complications and edge cases to the other.
The funny thing is, the same people periodically need one tool and the other, and even want to mix them. Many of them would consider having two tools for these jobs superfluous, especially if paid. They'd rather bend an existing tool, because how hard it is to add spans of colored text? or collapsed sections? It's, like, a hundred lines of code!
Hence the modern office formats, tolerably doing a huge spectrum of things, none of them brilliantly, and hopelessly complicated.