One explanation I read somewhere is that, by constantly pushing the envelope on form factor constraints and manufacturing complexity, they are making it impossible for knock-off companies to make convincing counterfeits.
with price competition, you go for cheap components, processes, marketing and distribution. when you compete on difference, you not only need new and unique qualities to market but a sustainable advantage.
one part of this advantage for apple is on the design and manufacturing side. apple maintains leads in these areas by being on the leading edge of what the technology can do (their products are never on the bleeding edge because their customers also demand quality and reliabilty). the other part is supply chain; not just the ability to source parts and get the best pricing, but the ability to create a cascade of innovation where components and design permeate the whole ecosystem (this lets them fully amortize fixed costs like r&d and manufacturing costs).
this is why apple captures the majority of profits in the markets it competes in. thinness is an attribute of that advantage.
It seems they're butting up against the limit of what they are capable of reliably manufacturing, which certainly won't end well. I'm looking forward to seeing what direction they take for the next Macbook Pro, should be interesting.