Do people buy the thinnest thing? Laptops or phones maybe to some extent, but TVs I sincerely doubt.
And having gotten to interact with the real process of product development, I gotta say in my (relatively narrow) experience it’s based a lot more on vibes/politics than market research or focus groups.
I can totally see “make it as thin as XYZ” being a hard requirement for no better reason than a PM felt strongly about it, and no-one had all three infinity stones necessary to call them out (engineering knowledge, understanding of the PD pipeline, and political capital).