Back in the day when people were building their own PCs from off the shelf parts, the heart of the matter was the BIOS ROM, which was a major stumbling block. Third party replacements like Phoenix and Award often contained the letters IBM in some innocuous text like, "this is not IBM " or something similar since code compilers looked for that in order to work. Apple has always had a very tight relationship between their ROM and the hardware, making cloning a non-trivial job. Aside from the possible cost saving at the expense of time to build, why would you want to? Could you build a better MAC than Apple?