[jayd16]

>a service where Google presents you an apk, and you sign it.

This doesn't work if the idea is to dynamically generate APKs for the vast Android ecosystem. In theory they could dynamically upgrade apps to be compatible with future OS versions etc.

[tdeck]

To add some more context on this, when you provide an app bundle the Play server looks at your device's display size, density, OS version, and architecture. If your OS version is new enough (L+ I believe), it can send you a bunch of split APKs. That means one big APK with DEX code, one with just the armeabi-v8a native code, one with just English translations, one with just xxhdpi assets, etc... In theory a developer could build all these split APKs, sign them, and upload them to Play.

However, on older devices that don't support split APKs, Play must compose and sign one custom "fat APK" with all of the stuff specific to your configuration, on the fly. There are a lot of different options for this (you can generate them using bundletool if you're curious). The upload size of all these redundant APKs alone would be a huge burden on developers.

This isn't to say that there couldn't be a way to do APK splitting while maintaining the integrity of the app signing system. My guess is that it wasn't a high priority to do so.