[wilsynet]

Relying on the App Store to distribute your app is more than a little different than building an extension to iMessages. Apple and Google want you to use their app stores in this way. Apple does not want you to bridge iMessages to other platforms.

[LeonB]

True, but I think this sentence buries the key point:

“Apple and Google want you to use their app stores in this way.”

…they want you to use their app stores, their way, because they retain 100% control of what you can and cannot do.