The more SKUs you have, the more digital shelf space you get on a shopping website. When your sole differentiating factor from your competitors is that your laptop has an "HP" logo on it and theirs has a "Dell" logo on it, your only effective strategy is to try to make sure fewer laptops with "Dell" logos on them show up above the fold in Amazon search results by creating lots of distinct SKUs to try to eat up as many of the slots on that first page of search results as possible.

Apple doesn't have to exist in that type of competitive environment. If you want a Mac, you're either getting it right from store.apple.com; or you're searching for Macs specifically -- in both cases, Apple owns all of the shopping screen real estate.