Because they produce glare for neighbors. Also tradition. Mirrors would be even better, but apart from expense and weight, worse for anyone in the path of the glare :-/ [edit: reply comment to this says mirrors wouldn’t actually be better]

But, for example, California is requiring light roofs and other mitigating issues, so there is some progress.

BTW if you do decide to paint your roof (or any other external surface) please use TiO2 paint as the UV from the sun will then cause it to break down pollutants like NOx and greenhouse gases like methane.