Native UI toolkits are not necessarily about looking beautiful for every app, they are about consistency, accessibility and letting the user expect how certain things work and can be found at. They have to fit to a lot of constrains that a single app doesn't have to.

Yes grey text on a white background can look great on a retina screen, but does it look good across the product range from the last 7 years and all devices, lighting conditions and for people with various disabilities?