One of the key goals in components is the ability to use it anywhere. Shadow DOM helps with that but also feels like a big thorn in the side for a lot of designers and developers to operate within. It makes more sense to design components this way when you're starting fresh and creating a library to be used anywhere. Controversially, I'd say it's often easier to just write CSS rules that fixes these conflicts and that the Shadow DOM is not really worth bending over for if you're designing within a controlled environment like your own product.