I assume it's because not every server (most notably: google's) support the XEP for offline messages, and having to always keep a connection to the remote serve

I assume it's because not every server (most notably: google's) support the XEP for offline messages, and having to always keep a connection to the remote server is mobile/battery-unfriendly. All these newfangled alternatives usually rely on the Google services' push notifications api afaik.