XEP-0198 is great for session management. But, Android and iOS don’t really want apps maintaining persistent sockets. WhatsApp solves this by leveraging the push infrastructure when the app is suspended.
XMPP also supports push notifications using Android and iOS infrastructure. But it also works with persistent connections, and the client notifies server when the app is not in foreground, for server to push only important data (such as direct message) to conserve battery.
XMPP also leverages the push infrastructure, because otherwise there is no way it would work on many modern mobile platforms.