To a large extent, I believe it's a holdover from the earliest days of the Mac, when the mouse was brand-new, and many developers had to essentially be forced to even support it, let alone treat it as a first-class interface device.

I agree that it would be nice if Apple would have more keyboard navigation available by default now than they did then, but given that it is readily available for those who want it, I don't really see it as a huge issue.