Line length is a worry because of readability. This is unrelated to the width of monitors. Having to read and visually parse a 200-character line is no fun. In addition, since most lines are relatively short, you also don’t want to have your source code window extra wide just in case, if instead you can use that space for other useful information and controls.

Also, why does your comment have a paragraph break after each sentence except after the third? Because of semantics and the related readability. The same is true for source code. Readable layout does not depend on syntactical structure alone.