It's already been said, but there are two major reasons:
1) Your monitor tends to be farther away from your eyes. Two feet at least. A "retina display" desktop monitor would have far lower DPI than a "retina display" for a phone you hold in front of your face.
2) Most desktop software isn't resolution independent. The reason Apple can get away with changing DPI so drastically on their mobile devices is that they wait until they can double it so that they can then pixel double applications.