The answer to these types of questions usually boil down to: Google wants to improve the web experience for everyone. They are the biggest benefactor of the web

The answer to these types of questions usually boil down to: Google wants to improve the web experience for everyone. They are the biggest benefactor of the web today, so it makes sense they have the most to gain the more pleasant the web experience is. Also I think they are looking to get revenue with their own Cloud services that integrate seamlessly with Flutter. For example Firebase is the #1 backend most Flutter Devs use.