First, Corporate boards are required to act in the best interests of the shareholders. Perhaps the single biggest example of a corporation that’s not fulfilling this duty right now and it being overlooked is Disney.
Second, Corporate boards are required to actually hold the CEO accountable. Again Disney is the key example of a company that technically doesn’t have a board because all the board members (many of whom are connected with other corporations) are all hand picked Iger stooges.
Disney is also a case study in why your argument for “the Shareholders should just toss out the bad leadership” doesn’t actually work the way you think - you should go look up the Disney corporate battle that happened earlier this year.
Third, Corporations operate under strict reporting standards, including disclosing expected company performance to the best of their ability. For example, they can’t just underreport in order to ensure actual results are better than expectations.
Fourth, rules that exist around corporate mergers and buyouts.
Those are what I know off the top of my head.