Well they don't really have only one of each category. They just market it like that and instead of giving variations a different product name, they treat it like the same product but with different attributes. For the laptops for example, they have different size models with different performance specs and hardware that changes over time. But they sell all of those options as a Macbook Pro. You might have a 2018 Macbook Pro 13", but by calling it just the Macbook Pro, they keep the number of models that users need to know about manageable.
I don't think it's just that they have fewer items in a category (they do) but that if you took all their SKUs for all products into markets, sub-markets, demographics, and ultimately individuals making purchases/using a particular SKU and organized it into a hierarchy - it would be almost a perfect tree with very few leafs touching the same branch.
Compare to Dell, whose equivalent datastructure would be a directed graph that probably contains cycles and non-sensical topology.
