But you can just look at the pictures to tell them apart. Oh, that one's tiny and made of aluminum, that one's bigger and made of plastic, and that one's bigger and made of aluminum. I didn't have to click through meaningless pages of non-differentiating differentiations to see those options, they were right there on the page.
For Apple it was due to a transition period. As far as the customer is concerned, the customer notices that there's a plastic MacBook, and it's cheaper than the aluminum one.