I don't know for sure, but I imagine it has to do with design sensibility more than functionality (like some other choices by Apple - eg aluminum phones even though it is heavy, gets dents and scratches and weakens the signal for the antenna... To be fair they do generally have good aesthetics). They probably believe, rightly or wrongly, that the buttons interrupt the appearance more than just a flat track pad or magic mouse does.
It could have to do with the OS, too... What is needed to navigate it. At some point i know Apple's OS didn't have right click (although I think they do now) so maybe just having one type of click (tapping on the track pad or magic mouse) suffices. I imagine apple has also find some research and discovered that people were tapping on track pads even when buttons were available, so they knew they weren't not including something must people considered integral... You have to remember that many of Apple's design choices are not geared for power users. The reason is they they only offer a few models. If you only offer a few models you have to aim for the average user, not the rare breed power user. Sometimes they include power user level hardware, like sick GPUs, nice screens, etc, but they won't let those power user features interfere with the design that will appeal to the averageish apple consumer - that's why you won't see something like a number pad on an apple laptop or buttons on the track pad. Those things are power user features that make it look less sexy, so they won't make the cut. If they had a more diverse line up they might have a computer that included those features.