Cracked Back of IPhone XR

I’m very careful with my phone and just noticed that my phone was cracked while I have a clear case on it. Brought it to Apple store and wants to charge me $400 for fixing it. I only have the phone in less that 5 months. No scratches on the side or in front, but the crack in the back is really big. Coming from the upper right going down. I really don’t understand how can I broke it like that.

I’m really disappointed. Bad customer service. Bad product quality. I’ve been loyal with Apple having 3 generations of phone used. But this one is really bad.