Alternatively thicker phones can be designed to be more robust and not need cases you know. Slim phones break easily because they have no affordance for robustness

That would be nice, but part of the benefit of cases is they're easily replaceable. I seem to drop my phone a lot; I've gone through two cases on my current phone already. The phone itself is totally unharmed.

you must have some very poor quality cases, I've never had to replace a case except for the flea market $2 TPU junk.