Because visual design is both a) very hard/expensive to do well from scratch and b) not a differentiator for most products. For the vast majority, it's just more cost effective to use a standard template and focus on your core product instead.
I would disagree. The designs I'm talking about are actually extremely polished. They spend an hour just to make sure the animation of a single component flows a certain way. And yet, they spend all this time and money only to make something that someone else already championed.