I started using in 2019, even though Firefox and Safari wouldn't support it for another couple of years, but Google's own Polyfill (of which I am a very modest contributor) was top-notch quality and so I had no problems using it in production for my LoB SaaS day-job. But my biggest let-down with the element is that it's comnpletely unstyled, beyond a very basic (and very un-Chrome-like) thick black line pixel border…
Another reason might be that vendor making it look like a native browser window would blur the line of death[1]. It would make it easier for malicious website to make a popup "browser update" in the middle of the page that redirects to seemingly legit Chrome download page and downloads modified executable.
[1]: https://textslashplain.com/2017/01/14/the-line-of-death/