They're unpopular because they're confusing and brittle. How is the relationship between the two maintained when the developer is hard at work, when the text is not just a set piece for pretty presentation? No answer is given, because there is none.
It's a bad idea because it's wasteful of space while achieving very little over a conventional setup.
The text editor or IDE can provide a feature to show(pop up) or hide(collapse) the side comment section when needed while formatting the comments to make them look bold or italic.
This will become a quality of life improvement if the implementation is done right. If not, it will be very confusing as you mentioned. I think writing API documentation can become very easy and since the function and its comment/description align on the same line, generating API documentation will become easy as well.
case closed, guys, we can all go home now.
Straight from my horror cabinet of ideas!