Apple's philosophy around high pixel density is to be able to run at a scale factor of 2. (Or close to 2 on mid-2010s Apple laptops, or sometimes 3, as on newer iPhones.) That way everything is physically the same size, but looks a lot nicer. Mac users running 5k monitors aren't running 5120 x 2880, they're running 2560 x 1440 at 2x.