Viewing distance is a big factor in perceived resolution. A Nexus 7 is about 300 DPI, and usually held a little more than a foot away from your eyes. A 27" desktop monitor 2560x1440 monitor is about 100 DPI viewed about 3 feet away. The resolution of both displays should appear similar. A roadside billboard printed at 10 DPI would also appear to be a similar resolution. If you are viewing a 27" monitor or roadside billboard at 1 foot, you are way too close.
There is no real need for a higher resolution display, just mere marketing. 4k displays also aren't significantly better compared to current 1080 displays at comparable size and viewing distance. Again, you aren't going to buy a 4k 80 inch screen and sit 3 feet in front of it unless you enjoy sitting in the first row at the movie theater.
I'm not sure that 3 feet is the normal distance at which someone would want to view a 27" monitor, but I should point out that your belief that there is "no need" for higher resolution displays and that it is simply marketing is from a vantage point of complete ignorance.
For a long time I used a 21" 1920x1080 display and I, too, felt it was more than adequate. Then I bought a rMBP before going to school, and now every time I go to the computer lab I instantly notice how much grainier and less sharp the screens are in comparison to my rMBP. And these are not cheap screens. The labs at my school are all outfitted with 27" 2560x1440 Apple Cinema Displays.
Go use a high density display as your primary machine--not just your phone--and I guarantee you will have the same experience. We can theorize up and down about perfect viewing distances and display sizes, but the fact remains that in practice a higher pixel density makes a significant difference in screen quality.