Why wouldn't you want users on modern smartphones to be able to zoom in the the same level of detail as is being displayed on large high-res screens?
Admittedly that's quite a theoretical argument, but: There's another factor besides image size and display resolution/pixel density, and that is viewing distance. When I look at a 4k 27" computer display I am probably about 80-120 cm away, my impression is mainly of the image as a whole and I won't primarily concentrate on small details. When I look at a 2k smartphone display, the distance is about 30 cm. If I can now zoom into a 4k image at native resolution, the relative size of details is much higher than in the other scenario. That's something one might not want, especially depending on the nature of the image. For a landscape image it might be different than for a portrait, where zooming in could reveal unwanted details.