Windows has supported high DPI screens since at least XP. I'm running Win7 on a 130dpi screen right now.

Some older apps don't support it nicely, but pretty much every major program made in the past 5-7 years handles it fine.

I think OEMs haven't offered them more because the consumers weren't asking for it. Unlike a phone or tablet, where you're using it 1-2 feet from your eyes, most PC monitors and laptops are at least 3 feet from your eyes. 96dpi is fine for that distance. HDTVs are even worse: a 1080p 50" TV is only 44dpi, but it's okay because most people are watching 6-10 feet away.