I have been thinking about the same argument recently as well and the most compelling argument I can come up with to justify it is that the states give up some

I have been thinking about the same argument recently as well and the most compelling argument I can come up with to justify it is that the states give up some of their power in the federal scheme in return for a degree of sovereignty. That conception of it works out pretty well if the federal government is limited and deals with concerns that all states share (like defense) and leaves most of the decisions that impact people's everyday lives to the states. IE, the degree to which the federal government impacts a person's life in California is far less than California's impact on that person's life. But interpretations of the interstate commerce clause really drive up the federal governments power to where that tradeoff in power might not be worth it. It is interesting, really, to consider this system where we have sort of two, dual sovereign entities governing us that are sort of kind of related but not directly so.