Oil and gas, if you ignore global warming, really is very, very cheap.
Furthermore, even if that were true there is nothing to stop a malevolent actor, either deliberate, by ignorance, or by lack of better alternatives, to just eat the cost.
No one does any risk analysis and the short-term upside makes the long-term downside invisible.
No one knew how problematic pesticides can get. We are just now discovering the effects.
It's particularly severe when it's up to an MBA running a company, but an engineering education only goes so far in mitigating that tendency: the Koch brothers majored in chemical engineering at MIT. A major component of their education was all about externalities. And yet they've devoted their political activism to nurturing anti-intellectualism in the Republican party specifically so that the GOP would deny any and all externalities.
Don't forget, a lot of engineers don't even consider software engineering to be an engineering discipline.
I still have an extremely conservative outlook, and I have no ideological opposition to the private sector, but when "conservatives" are the ones running around upsetting apple carts and setting fires, it becomes hard to be an engineer and a conservative.
The CEO of, for example, Monsanto, has doubled down on Roundup in so many ways he cannot fail to fight this and keep his job. So I'm not surprised that he is fighting it (and getting Monsanto's many PR resources to fight it). But, the longer term for Monsanto is very negative if they don't change course, because even if they can suppress this in the U.S. they can't in Europe or elsewhere.
So, while the failure of our regulators and/or legislators to respond is certainly part of the problem, the other part is that our corporate boards of directors, whose only remaining job really is to know when to pull the CEO, are not up to the task.