That's true, but in many respects the interesting physics really does become 2 dimensional at that point. Conduction means electrons (or holes) moving through the material, and in that sense, there is one less degree of freedom to their motion. Even thicker materials can exhibit essentially 2-D electron behavior. So, while the materials are obviously three dimensional, the physical properties of interest are those of a 2-D system. (Weird, but then it's a quantum system so what do you expect?)