Fifteen years ago, I had a Garmin GPS (admittedly not a phone, but similar form factor) that survived a week of knocking around the bottom of a raft.

The battery compartment had a rubber gasket and some very tight screws.

How much of the total volume of the device was the case/housing?

I suppose the glue-everything approach is partly due to the desire of making a device very thin. There's no room for strong, load-bearing outer case, the internals are load-bearing.

You just need well designed rubber gasket. Thickness is impact resistance thing in those devices