Are we talking about the height of the phone here? The wide dimension bezel of the phone is already as small a bezel as can be, maybe a some trivial size reduction there.With a thin display bezel, they can make a thin casing bezel.
I haven't seen people holding the phone on top and bottom bezel, I don't think the slim bezel on top and bottom can be an ergonomic issue.
The only issue with the height that I can think of is pocket-ability of the phone. Another reason is probably psychological - aesthetic reason, want the phone to look cool. Is that why people demand thinner bezels, because it might look the so called 'cool'?