Pixel density affects many aspects of a system;
Storage - it affects the size disk you need/want because high resolution images / video take up much more space.
Memory - You need more memory to build up screens for a higher density display. Further you need the bandwidth to shove that data around.
Compute - If you want to 'render' to the display rather than just copy bitmaps around, or composite complex bit maps, you need to spend a lot of time computing which can make other things slow.
So when you look closely at tablets you will see interesting places where they have been adapted to support these densities.
But more importantly there is 'change' in the systems where there is new money being invested. So tablets are getting all of the 'change' now, less so with laptops, and hardly at all with desktops.
The reason this will change though is that I expect we've convinced display manufacturers that 'regular users' (the bulk of the purchasers) want 'high dpi' displays. Its not easy communicating with an entire industry but success Apple has been having with 'retina' displays, and the more recent Android tablets with higher resolutions, means more people will jump in to support them. And more importantly when the choice is available folks reject lower density displays. So in the great 'tuning' these guys do where they calculate how to get the most money out of each hour of running their factories, the equation is tipping in favor of high dpi displays.
That said, I'd love to have a couple of 32" 2560 x 1600 displays for my desktop, but I think that is still a couple of years off from being 'mainstream'