Because the 1440p display on the Samsung is not a true 1440p display. It uses a diamond pentile 1080p AMOLED display, where each of the green subpixels is basically cut in half so they have separate controllers. If we're talking actual display, it's 1080p. If we're counting by number of pixels, it's 1080 and a third. It's nowhere close to 1440p in actual number of Pixels no matter which way you cut it.
It achieves 1440p-like viewing clarity by virtue of the half-sized green pixels. They light the pixels up to different brightness, with some of the green pixels brighter than all of the other pixels to create a natural antialiasing effect that masks the "jaggedness" when looking at the images closely creating a semblance of a 1440p display.
We know what happens to AMOLED when you put them at relative different brightness to their adjacent ones. They degrade faster. While this won't create burn ins because the degradation is nearly screen-wide when setting this to 1440p, this will lead to an earlier color and total brightness degradation point compared to when using the device at it's true native resolution of 1080p, which will retain the color and max brightness point for longer time since it's less degraded and well, it degrades evenly.
Of course this probably won't matter if you replace phones year after year, but then it could affect the resale value if you're a very heavy user and degrade it so bad quickly. It will also matter if you want to keep the phone beyond the 2 year mark.
I keep my devices from Samsung at 1080p because I want to take care of it as best as I can. I don't particularly care for the display resolution at 1440p anyway, because I can barely tell the difference. The resolution is one of the least concerns in specs I have when looking at the phone. If it's 1080p, it's good enough for me.