Like people mention above, the hardware isn't the only impediment; Apple's control is what helps them a lot with this. Desktop Windows' support for DPI scaling

Like people mention above, the hardware isn't the only impediment; Apple's control is what helps them a lot with this. Desktop Windows' support for DPI scaling is anemic at best, and this is only partially fixed in Windows 8 by having ad-hoc dimensions per-device, or on PCs just "big" vs. "really big" mode.