Lots of reasons. The most obvious ones that come to mind:

1. People like a variety of custom cases that themselves have features (eg wallet cases random designs etc). If it’s built into the phone that customization capability is worse because you now have two layers of protection making for a very thick and heat-insulating design.

2. It’s valuable to have partners that make accessories for your device. If you kill that line of business for them, other things may go away and those partners will want to work with you less.

3. An integrated case will still suffer cosmetic damage. But now without the option to replace, you’re stuck with that damage.