>>what stops companies from ruthlessly exploiting workers, selling products that damage society, polluting the environment, et cetera ...?
When the companies engage in the ordinary big-business practice of regulatory capture, absolutely nothing.
It is just treated as a game to be hacked. And hack it, they do.
Because according to your standards, companies don't need to treat everyone fairly (as a good citizen), but the govt must treat them fairly.
That's how earlier this week we saw headlines that Tesla employees in Buffalo NY were organizing a union drive. Then the very next day, we see headlines that Tesla has fired 10 workers, the ones organizing it.
It is illegal as hell, but they did it anyway. Why? Because Tesla knows that it'll take years for all the hearings to work their way through the courts, and the result will be a fine, which they can appeal, and delay further. Meanwhile, they figure the end cost will be less than having to deal with giving workers a fair deal. So, despite all the "excess govt regulation", the workers get exploited and screwed.
It is also how Norfolk rail donated to both Trump and the Ohio gov and had regulations eased, e.g., requiring fewer workers on trains and not needing to update their technology with "extra" brakes. And now we have an environmental disaster as a result. And those costs will be 'socialized' to the taxpayers and victims in the town, while the company keeps most of it's profits (it will not pay even a small fraction of the costs and it will be years from now).
So yeah, the govt structurally CAN NOT be as nimble as the companies, so the companies exploit that weakness. Great for them. Sucks for society.
I'm not saying they're all bad or evil, but this over-simplistic "shareholder value only" concept /structure is not good for a sustainable society or even for sustainable corporations.