Except then you have to use Dart, or call into Dart from some other language. There are many people who dislike Dart or otherwise prefer to use other languages.
Any multi platform framework will inevitably hit the hurdle of supporting only the common denominator. For many applications this is fine, but for the sake of consistency I hope native applications will still live for a while.
Having worked with React Native and Xamarin: wherever platform specific problems come up, there is some sort of "escape hatch" you can use to tailor to it.
Still room for improvement but it's not so restrictive about the common denominator.
Google is a web company ( at least in their heads ) and Dart was supposed to replace Javascript ( lol ), they used GWT for their projects since forever and now many use Dart and Angular - Adsense for sure and many other very important products for them ( $$$ ) so it's important they capture the Web side of things with Flutter because Google and their PMs and VPs are the most important clients in order to ensure the future of Flutter.
Please note I didn't mentioned anything technical, it's pure product management and strategy and that's one of the reasons I'm optimistic about Flutter's future.
Honestly I seriously doubt Flutter will ever be popular for web. It recreates everything already included in browsers like DOM, CSS, text editing, etc. There is already too much bloat with modern JS apps.