Battery saving mode can accomplish that, if you will.

Some apps, however, need to stay “awake” to catch notifications or do some background processing needed for their usual functionality.

Imagine if you were using a navigation app, put it in the background, and after some time, put it on the foreground and it had to start the whole process of locating you and calculating a route to where you were going.

That being said, most apps don’t really need to do anything in the background, except for collecting data about your behaviour.