Because they're expensive. It's the same reason that phones have higher DPI than laptops/tablets do -- what's economical on a 6" screen isn't on a 13", and what

Because they're expensive. It's the same reason that phones have higher DPI than laptops/tablets do -- what's economical on a 6" screen isn't on a 13", and what's economical on 13" isn't on 27." (Why? Yields. A 6" phone screen is like 12 square inches. A 27" desktop monitor is over 300 square inches. If you have a 98% yield of phone screens without defects, that implies only one defect per ~600 square inches, which means that at a handwave level, you'd have a 50% yield of desktop monitors with that same tech. "We have to throw away half the screens we make as junk" is a lot worse than "we have to throw away 2% of them.")