Humans can be real bastards and governments aren't magical solutions just as corporations aren't magically evil.
The question is about who can be held more accountable, who can be more feasibly removed from power and what kind of institution can be more promptly remedied.
We as moral actors could potentially change the laws of governance probably far easier than we could form a corporation to defeat ExxonMobil in the marketplace. Both should be easier, but that's another discussion.
Both governance and private capital are imperfect and both deserve criticism. Being a fan of either is a mistake.