On most platforms it's quite easy to embed a browser in a frame (show a changelog, an email, or a page of interactive charts). With a few tweaks this can feel completely seamless.
It becomes really painful (or impossible) though, if you need those complex text rendering on multiple places scattered over the native UI. Or if the native UI should interact with the HTML somehow (drop-downs, edit text, add native controls inside html).
Thats why everyone is building Electron/etc apps.