You don't need to abandon plain text to hide comments. Comments are detectable with a regex; an extension to hide comments would be trivial to make in most editors. I think it's not a common feature because people generally just don't want it.

Rich text code would create so many problems, too. You get locked into a special editor. You need special version control. Grepping becomes difficult. Diffs become difficult. You'd need a whole separate ecosystem, and for what? We have treesitter; we can already treat code like data.