Because of how long our arms are and how far we can reach. It's all about the interface. The same is true for computers. Different manufactures use different hardware, some chips are faster than others, and some hard drives are bigger. When we talk about computers, though, and specifically when we brag about the ease of use of a Mac, we're talking interfaces.
Software design is the key to a successful interface.
When the OS and the applications are awkward, getting in the way of what we want to accomplish, the interface needs improvement. This is what originally drew me to the Macintosh interface when I began working in computer music. My first class in 1987 used a Mac, as has every class I have taken or taught ever since. It was, and still is, the simplest, most elegant, and most transparent operating system in production. This is the reason for both the incomparable loyalty of Mac users and the strong connection Macintosh has with creative artists.