I am a mechanical engineer. GP is correct: if the skin stays the same thickness, then halving the distance between the two skins halves the strength to resist bending. Central reinforcement, like an I beam, don't help with the bending force but rather stops the two skins from moving towards or away from each other, AKA buckling. The other way it helps is as a shear web, in that it stops the skins from sliding past each other.

But basically, for a laptop to be half the thickness and equivalently strong it will need the structure to be twice as heavy, as the skins and webbing need to be twice as thick. That leaves even less room for battery, quiet fans and replaceable components, a high price to pay for something which is labeled "Pro".