[Alexendoo]

There is quite a large downside to app bundles, that is that it requires you (the developer) to hand over your app signing key to Google. The fact that updates from the play store are signed end to end is pretty great in my opinion, this doesn't apply so much if you give away your signing key.

As far as I can tell it's not a technical reason why they ask for your key - bundletool [1] supports generating a set of signed APKs (.apks, APK set archive) in the same way the play store would from an app bundle. Android Studio's GUI could just as easily generate that to upload directly to the play store instead of an app bundle. This would net the same benefits to the end users and the same level of convenience to the developer, all whilst keeping your private keys private.

[1] https://developer.android.com/studio/command-line/bundletool