You don't need to put 12 million pixels on your head. 12 million pixels is useful on the desktop because it's nice to have a high density display anywhere you look.

For an HMD you only need "retina" resolution straight ahead. Peripheral vision can make do with a much lower resolution. I have no idea what the magic number is for retina HMD, but it'll probably be smaller than 12 million pixels. It's certainly possible that the 5" 1080p display used in the latest HTC could have a higher apparent resolution than putting 3 WQHD monitors on your desk.

And also keep in mind that higher densities are much easier to manufacture affordably in smaller screen sizes. There's a reason why you can buy a phone with a 440PPI, laptop displays with 220PPI but it's hard to buy a desktop monitor with more than 110PPI.