One could also ask the question "how many people would prefer to have a phone that didn't --need-- a case"?
Given the choice between a thin phone that needs a case and a phone that is as thick as a thin phone with a case that is more durable, possibly has a headphone jack and a slightly larger battery, I would personally go for the latter.
If a phone is designed so the screen always breaks when it hits the ground unless it's protected by a case, it would make more sense to manufacture the phone inside a case already, no? I.e. make a raised rubber bezel around the screen's edge part of the standard design.
I've never had a phone come with a case? Strange. But these days I just use the 10$ minimal rubber cases. It's enough to protect were the screen and case meet which is the source of all my breaks.
I’d never used a case until I got the iPhone 8 and had to buy one because of how slippery it is! Unless you put it on a perfectly flat surface it will just slide off on to the floor.
Given the choice between a thin phone that needs a case and a phone that is as thick as a thin phone with a case that is more durable, possibly has a headphone jack and a slightly larger battery, I would personally go for the latter.