I definitely have to agree, but they're getting serious competition on that front now. The Surface Pro is very close in ppi to the rMBP. Vendors are asking for this to compete with Apple.
PPI isn't everything. The quality of the glass/plastic is also an issue. My DELL 4K monitor certainly has the resolution, but it also looks like I'm staring at a textured surface because they use cheaper acid etched plastic as the anti-glare solution.