That seems like a good question to pose about the person who is leading up the “Department” of Government Efficiency (DOGE). After all, it would be reasonable to expect that a government agency committed to increasing efficiency in government would go about its work in an efficient manner. It would be pretty hard to make that case about DOGE.
No they are not. It depends entirely on the corporate charter, though most are set up that way. Additionally, there are non-profit and not-for-profit corporations.
The idea that corporations’ mission is solely to maximize shareholder value goes back to Friedman and the Chicago spivs. There were corporations for centuries before that bullshit became current.