How come seven people (the supreme court) can have so much power?

I am not American but it seems to me that it is an incredibly broken system that 7 judges can essentially halt an entire country's progress. They decided that corporations have rights like a person, they can decide if gay marriage is legal, they can decide basically anything if they wanted as I understand it.

So why does this even exist? Surely such gigantic decisions should be left to a parliament or something.