I'd guess that the main reason Googel has done this is to prevent side-loading of messenger apps, such as Signal, with true end-to-end encryption. Such messengers would be very difficult to surveil at scale. You might ask why not to simply install these apps from Play Store? The reason is Google demands signing keys for all apps, so it can impersonate the developer, inject any spyware, rebuild the app, sign it and make it look untampered. Side-loading bypasses this entirely.

Google doesn't demand your signing keys, and transforming your app in that way is easily detectable by the developer.