It's not meant to be an editor. That sounds like unnecessary feature creep - the point is to run snippets of code in different languages through your web browser, not to be an IDE.
I think syntax highlighting makes sense. People do write code right into things like this. In that case, syntax highlighting helps if that's what you're used to. Also it is helpful even if copy pasting for readability if you're normally used to syntax highlighting.
But people still need to type those snippets. Typing up fresh code in online code runners (as opposed to pasting in pre-written code) is extremely common.
(Disclaimer: client is by me) but I have an alternative front-end for TIO which supports syntax highlighting and indentation for popular languages: https://vihan.org/p/tio