It is apple though? Don't consumers prefer lighter and thinner devices that do what heavier and thicker ones do?

It's Apple. iPhones, for example, are unusable and slippery without a case, after which they're as thick as high-end Androids with Kevlar backs that can be used without a case.

If my iPhone had good grip surface I wouldn't use a case, and the thinness would actually make a difference in my experience.

That is your opinion. They’re not unusable or slippery for me. I’ve never had a case on any of them, never dropped or cracked one, and get a lot of enjoyment out of the thin and sleek form factor. Your problem can be solved with a case. A thick phone has no solution to make it thinner.