Why would GM build both Chevrolets and Cadillacs?
Market segmentation. Apple gains nothing by producing a Linux line since Apple is already using a certified UNIX platform. All it would do is over-complicate Apple's offerings and massively add to integration, testing and support costs. If Apple decided to segment in this manner, it would be easier and less costly to produce a down-market variant of the current BSD. Even Chevys and Caddies use a lot of the same parts, an Escalade basically being a Tahoe with a facelift.