4K is an outrageously high resolution.

If I was conspiratorial I would say that 4K was normalized as the next step above 1440p in order to create a demand for many generations of new graphics cards. Because it was introduced long before there was hardware able to use it without serious compromises. (I don't actually think it's a conspiracy though.)

For comparison, 1440p has 78% more pixels than 1080p. That's quite a jump in pixel density and required performance.

4K has 125% more pixels than 1440p (300% more than 1080p). The step up is massive, and the additional performance required is as well.

Now there is a resolution that we are missing in between them. 3200x1800 is the natural next step above 1440p*. At 56% more pixels it would be a nice improvement, without an outrageous jump in performance. But it doesn't exist outside of a few laptops for some reason.

*All these resolutions are multiples of 640x360. 720p is 2x, 1080p is 3x, 1440p is 4x, and 4K is 6x. 1800p is the missing 5x.