Old versions of macOS / AppKit used to use WebKit to render rich text inside their native NSTextFields. Turns out text is hard :)
And besides, the native WebView is super fast and lightweight, and its not unreasonable to use it as a text layout engine. You could use separate webviews for every row in a table and you'd still get fantastic performance.
iMessage for mac used to use a webview too. Adium as well. HTML is absolutely the right tool for the job if you're rendering rich/marked-up text.