I don't care about 4K. Apple displays have had resolutions higher than 1920 by 1080 for a long time. But why don't those higher resolutions come in monitors that the average joe can actually afford??
1080 is often manufactured from panels shared with the television market in equivalent sizes, and televisions span a very broad range of sizes. You typically go past either 1920x1080 or 1920x1200 when you go larger than 24". Almost nothing below hit that until recently. They aren't all unaffordable though. The monoprice and unbranded Korean import 27" displays were only around $400.