Because there's never a guarantee the app is open, especially on mobile. If we take your Snapchat example , if one user is trying to send a message to another

Because there's never a guarantee the app is open, especially on mobile. If we take your Snapchat example , if one user is trying to send a message to another and their app is closed or their device is off, where does the message go ? On the desktop you have the luxury of mostly running in the background to pick up the ping, but it's almost never the case on mobile.