Why do teeth turn yellow even if you brush every day?

The surface of your teeth is very similar to porcelain. If you were to look at a porcelain surface (a teacup or similar) under a microscope, you would actually see a surface with cracks and crazes (fine cracks) that under even higher magnification would be like a network of valleys and canyons where food can collect. Zoomed out, this network of valleys and canyons filled with food debris looks like stain. This happens over your lifetime, causing your teeth to appear to gradually become more yellow, because of the food debris trapped in the grooves, where a toothbrush cannot reach it.

This is why food that would stain a teacup (coffee, tea, red wine, turmeric/curry, etc.) can cause your teeth to appear to stain faster.

Bleaching or whitening the teeth uses a strong hydrogen peroxide-like chemical (carbamide peroxide) to clean out these grooves and dissolve the trapped food, causing the teeth to look lighter and more like they did before the food debris got caught in the grooves.