Comment by friendzis

1. It's easier to design and build Ingress Protection without user-accessible compartments.

2. There's a lot of tech on the back: NFC, wireless charging, structurally important [magnetic] attachment points. Ensuring electric contact and physical strength on a door is again hard and expensive or all that tech has to live on the battery.

3. Design. A glass-like openable door is going to be extremely failure prone.

4. Compatibility. You can't guarantee quality of 3rd party batteries, even more so if the tech is in the battery pack.

5. Planned obsolescence. Let's not kid ourselves, encouraging replacing the whole phone is good for business.