> They could have started ringing alarm bells and used their massive strength to push for environmental research and favorable regulations.
i.e., acting against their own interests. Spending the last 40 years using their strength against environmental research meant that the people who in 1980 owned and managed Exxon extracted great benefit to themselves (at the cost of others) for the rest of their lives (either they're already dead, or will be before any serious consequences may hurt them); doing what you propose would have benefitted the world but greatly hurt their own interests.