You need to open a connection, which requires user action. When both devices are already connected to the cloud, the synchronisation can be done "transparently"

You need to open a connection, which requires user action. When both devices are already connected to the cloud, the synchronisation can be done "transparently", which is what users today have come to expect. Another reason is that if you still want to sync with the cloud, you need two synchronisation systems that have to be kept in sync too.