I think that it's not 120 is thinner, but that 35mm is thicker to prevent it from tearing out the perforations, you could probably coat it on the same base if the base was polyester so that it wouldn't tear so easily. sheet film base is thicker still to give it stiffness needed so it doesn't slump down in the film holder. A thinner film would require a different focus adjustment just like a thicker one would.
You must be blocking me and others. We explained the reason! It is that 120 is thinner due to the backing paper.