500W of cells is about 1kg with another kg of resin on top if there's a surface to bond them to. And costs about $100
Like anything with cars it's purely a matter of fashion, ego, and perception. Noone who isn't making a purely emotional decision buys a new car. Adding a thousand or two and 5kg is completely irrelevant. If people don't percieve an extra 10-20 miles a day as worth the ugly blue rectangles, then the one with them could be 200kg lighter and $5000 cheaper and it still wouldn't sell.