Maybe I am stupid, but can someone explain to me the point of even having a President/Federal Gov. in the US if the states can do whatever they want?

Maybe that's not the best way to word the question, but I will try to explain what I mean. So the president always gets blamed for things like high gas prices or inflation, but this is not the case in every state. Some states are better off then others. That's why people move. Some states have more jobs, less inflation and lower gas prices. Some states have legal marijuana, abortion and more LGBT rights, while some don't. There are state laws that conflict with federal laws. But the way I see it, is the federal government seems to have very little influence over what the States do. So I guess my question is, how does the Federal government actually impact the nation as a whole. I get things like war, foreign policy and trade, but if the President and federal policies really have that big of an impact as people claim, why are some states better off economically than others?

I was watching a video about a debate on how the electoral college isn't the will of the people, and that true will of the people would be determined by popular vote. The guy in support of the electoral college was arguing that it's necessary so that each state has a voice. Okay, so if that's the case, if the states have so much power, what's the point of even having a federal government?

He gave the example of how in France and Germany for instance, the entire country had the same COVID mandates, while in the US each state had their own, and he thought this was better. So you tell me who it is that has more power in the US?

Maybe I am stupid, but can someone explain to me the point of even having a President/Federal Gov. in the US if the states can do whatever they want?
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