Sometimes I play Sky Force Anniversary when I'm doing nothing. But, when I open the game in offline, it redirects my to Play Store and few seconds later it shows that I dont have any internet connection. Why can't I play my offline game in offline?
Because the DRM software in the game requires an internet connection to verify you've genuinely purchased the game, every single time you try to use it.
This allows the company that maintains this software to take it away from you at any point, simply by taking it's verification servers offline.