If all browser tabs/apps ran even in the background, it would absolutely hammer the battery life of the device. So operating systems and browsers are very strict about what can run in the background, for good reason: saving battery. Nobody wants an app to drain all their battery and many users will uninstall any apps they find wasting battery (and possibly give them a bad review).
The question here is, what are you trying to do?
If you want to play music in the background (e.g. music player app): you can enable playing in background on the Audio object.
If you want to simulate real-world time passing in the game: wait until the app wakes up, measure how much time it was asleep for, and then simulate that much time passing. That has the same effect and doesn't waste loads of battery.
If you want something else... what is it you're trying to do?