It can be both, and often is. Honestly, I'd be surprised if Apple's continued push for thinness comes from just a single team. Design pushes for thinness, because hey, it looks pretty damned impressive. Engineering eventually figures out a way to make it work after (I presume) screaming a bit. Which is impressive.
Marketing and the business side love it because it's a concrete metric they can point to in order to justify it to consumers and it doesn't require any specialized knowledge for consumers to understand. Thickness is simple. Everyone can understand it, and some people can even use a ruler to measure it themselves.
And consumers continue to choose to reward them by buying thinner devices. After all, if we can't have flying cars, personal jetpacks, and live like The Jetsons right now, we damned sure can have thin devices as a consolation prize. Consumers would probably buy these devices even if they weren't thinner than the generation they replace, but it all gets tied together. Not to mention the pesky problem that the biggest problems (lack of repairability, keyboard switches that are deathly allergic to dust, etc.) aren't really noticeable until something goes wrong. And then, it's "my MacBook is broken" and not "my Macbook is broken and the ultimate cause is a zealous focus on thinner and thinner devices."
Marketing and the business side love it because it's a concrete metric they can point to in order to justify it to consumers and it doesn't require any specialized knowledge for consumers to understand. Thickness is simple. Everyone can understand it, and some people can even use a ruler to measure it themselves.
And consumers continue to choose to reward them by buying thinner devices. After all, if we can't have flying cars, personal jetpacks, and live like The Jetsons right now, we damned sure can have thin devices as a consolation prize. Consumers would probably buy these devices even if they weren't thinner than the generation they replace, but it all gets tied together. Not to mention the pesky problem that the biggest problems (lack of repairability, keyboard switches that are deathly allergic to dust, etc.) aren't really noticeable until something goes wrong. And then, it's "my MacBook is broken" and not "my Macbook is broken and the ultimate cause is a zealous focus on thinner and thinner devices."
So here we are.