Because the general public that doesn't know any better and continue to think bigger is better, and so manufacturers will always use that as a selling point. All flagship models these days can shoot UHD. As previously mentioned, it's all about aperture settings these days and perfecting low light quality. Which btw, all manufacturers are getting better at. Each new device that has come out over the past year (Whether it's Samsung, Moto, LG etc) has improved their camera quality over the previous model, without significantly raising the MP.
S5 tap'n