Why are graphics workloads float32? 32bit (1million+alpha) which is higher color resolution than most eyes can see - "true color" - is 3 8-bit ints + an 8 bit alpha channel (sometimes)
GPUs are not (only) about representing pixels, they are (mostly) about geometric computation.
Because before you can see a color, you have to compute it. For example, you need to calculate what color would result from the interaction of a light source of a given color / intensity and a surface of a given color / reflectivity / glossiness etc. There's no way to reasonably compute that using just 8-bit ints without getting terrible banding / quantization artifacts.