I'd say its a consistency thing. Apple values UI consistency above almost anything else (differences between their annoying metal theme and regular aqua widgets not-withstanding). They want the sizes of icons, text, and images (and even printed output for that matter) to be similar if not the same on all of their systems so they dictate screen resolutions that result in between about 72 and 95 DPI.

I'd like a high res powerbook too (although 1024x768 on my 12inch isn't too bad), but I wouldn't count on apple delivering it unless your willing to hookup an external higher res monitor.

-Mike

Edit: s/rez/res/g to preserve tonyc's sanity