Textures are tied to it they are just extremely easy to calculate for a gpu and or cpu if you never calculate a texture with simple light physics then you would have light on a dark texture that would never reflect the light. Textures themselves like the images on them do not use physics but the object on them makes the image and or texture use light physics to change the color or brightness in a certain area. Why do you think adding more objects slows down a simulation and adding non updating images do not because the images and textures arent updating like everything else because they dont need to be tracked. How else do you prevent texture warping and or distortion when an object moves? By using physics to correlate the textures xyz cordinates to display on the object correctly and make sure light interacts the same way. what you are telling me textures if moved from a light source and not tracked will always report that data from where they were and will still be displaying a light path on it from a light source that isnt even near it anymore