Why must I except a charge for an app that doesn’t work?

I installed an app on my phone but the app that was needed to make things work for my Mac wasn’t found in the App Store. That was the first sign something was up. After installing the app needed for my Mac I paid for the in app purchase believing it would allow the addition features to work with my Mac since it never worked as promises. So I uninstalled the app from my phone and my Mac. The final charges are paid and Apple refuses to authorize a refund. Now I can’t get my money back. Why?