Moreover, these kinds of UIs are still possible with the 'flat' look that's in vogue today, so there's little excuse why others choose not to do it. Perhaps one reason is that basic auth lost out early on to site-supplied login forms, so people got used to entering usernames and passwords into the page content anyway, instead of the browser UI.
For the most part, basic auth only tends to affect uses like intranet sites, router login pages, web services, remote management pages -- settings where phishing can still cause (serious) damage, so a harder-to-fake UI would be beneficial nonetheless.