What you call underfeatured many would call correctly featured. A more complex format doesn't add value for most use cases, while being harder to reason with and correct issues in.

.md files avoid the copy paste font/size mess by being plain text and rendering in the reader's choice of font. Bold, italics, hyperlinks and such are all explicitly added, easy to Ctrl + F for and aren't hidden behind finicky context menus as in standard word processors.