Why do you think that? It's not how it's defined. In fact it makes no real sense to call 2560x1440p "2K". We call 3840x2160 "4K" because the horizontal axis is pretty close to 4,000 pixels. So the only TV resolution it would really make sense to call "2K" by the same logic is 1920x1080.
It's the marketing term. It doesn't need to make sense