Yes, shareholders can sometimes exercise power over a company via its board, but it doesn’t follow that shareholders necessarily want to maximize the value of that company’s shares, or at least not right away. Shareholders often have other interests. For example, they usually own shares in other companies. Also, ESG is big these days.

This can make deciding what a company should do a messy political process when there’s disagreement about what shareholders want or should want. Like, some shareholders may decide to buy a coal plant to shut it down, others to keep it running as long as possible.