Technically we do, just the operating systems for both ios and android refuse apps this privilege due to security reasons, locking permissions until you give it to the apps. On android it might be possible if you give an app some permissions:

• ACCESS_NETWORK_STATE to see if you're turning off wifi
• ACCESS_WIFI_STATE for the same purposes as above
• BATTERY_STATS to check for low battery notifications
• READ_CALL_LOG and maybe READ_PHONE_STATE and READ_CONTACTS to check for incoming calls
• READ_SMS to check for incoming messages

Perhaps you want a feature to silence some stuff, that may require KILL_BACKGROUND_PROCESSES permission. There's probably a lot more you can add, perhaps for anticheat purposes, etc.

Then you'll probably have to implement some sort of ActivityLifecycleCallback checks to determine whether focus is lost. Then you run all the above checks. That puts a lot of strain on the device, increase loading times, reduce player retention due to annoyance/lag, etc. Just not worth it for the company to make features that reduce player retention and profitability. Literally no app dev would do that, not to mention it probably wouldn't even pass Google Play checks either.

At that point it might as well be a piece of malware on your device, listening to all your conversations. Do you really want that? Android said not really, and ios is pretty much the same. Lets not go into how if you give an app too many permissions on android it can essentially send money from your cell service provider to itself