This is by design: the project came out of Inbox.

The thinking was that having portable widgets was not a good thing and for the highest quality app you still want native UI, while having portable "model" [ as in model in model view controller ] code.

Flutter was developed by a different team, and has a different philosophy about this - and in part this is driven by solving for a different goal. For super sophisticated complex apps with large teams, native is still the way to go. If you look at all of Google's own major apps, they are all native.