Because resources are scarce. If solar panels were free, then yeah let's plaster them everywhere. But let's start with where they make the most difference.

Because very few people have covered every last spot on their roof so you would be designing a product suitable for very few people

Or for people who don't own a roof at all - plenty of such people in cities.

Those people also have their cars in multi level car parks with no sun so that doesn't work for them either. In that case they would be better having the car park have solar and the local government building solar farms. A panel on something almost always undercover makes no sense.

I live in an average residential street in Europe full of 3 story residential houses. Our house with 6 apartments has 3 garages, the majority park on the street.

Sure, the government or the electricity company could just install car chargers and put a solar plant to power that in some rural area. But if I want something now then buying a car is much simpler than lobbying for that.

Very few people have electric cars to begin with, and they're really expensive compared to ICE vehicles so you're already talking about a demographic that's demonstrated a willingness to pay a premium for driving an electric car.

If Tesla offered a "Solar Panel Kit" that made their cars even a few percent more efficient it would sell like hot cakes.