I don't Android, so I really don't know - but are those PWAs the only or primary options for most users on the Android platform?
If not, they're really just "native apps which are also available as PWAs", and without data on relative adoption rates it's not really very useful information.
I use web apps all the time, even on mobile. And I'm sure I'm not alone.
Speaking of which Fastmail's Android and iOS apps are just the web app packaged in a native shell ;-) and they are not alone. For them clearly it was cost efective to work on that web app.
As for why they bothered to package their web app in a native shell? Maybe that's were the problem is.