Needing to run a full-fledged browser in the background in order to display your html/css frontend adds a lot more performance cost than necessary, making the app eat up far more RAM and CPU than necessary. It probably also introduces a lot more security vulnerability concerns that an otherwise simple app shouldn't have to worry about. And then there's the dependency chain you're introducing -- now your app needs to be updated every time the underlying browser gets an important update ... and maybe needs to be tweaked/rewritten to accommodate that browser update if it changes the way the browser interacts with your app frontend.

There are plenty of other GUI frontend frameworks that are also expressive, simple, and well-known, without all of these potential problems associated with them.

The problem is that most of Lemmy is old and afraid of change. They look back on old style applications with nostalgia while completely forgetting all of the issues from that era.