The materials are more expensive and heavy than what car roofs are normally made of, and the charge they would generate is miniscule. It may not even offset the added energy needed to move the car because of the added weight. Particularly if you live far from the equator, or somewhere cloudy, it’s probably not worth it.

When I was a kid (in the early 00’s) there were solar cars on TV and they were always these absurdly shaped pancakes made of ultralight materials and couldn’t even reach road speeds. I’m sure the tech has improved since then, but the real innovation that made electric cars possible was batteries. It’s hard to generate enough energy on the same platform you need to move without it being too heavy.