Define “often”. Did you know that Elon isn’t chairman of the board of Tesla?
Also no, chairman isn’t only accountable to themselves. Where did you read that? Investor having seats on board is so they can have a voice in the direction of the business.
Activists investors often try to have a minority stake and a board member so they can force changes at the company -[0]. A recent example is Elliott’s effort at Southwest Airlines -[1]
Often as in much more than 50% of the time for fortune 500 companies. If the CEO and the Chairman of the Board is the same person then there is sort of a conflict of interest. The Board is there to oversee the work of the CEO. The board is headed by the Chairman of the Board. Thus, if the CEO and the Chairman is the same person, then the CEO is accountable to himself.
Not sure why you include investors all of a sudden, as I didn't mention about that. I have no issue with shareholders having a seat on the board.
Which is kind of weird and illegal in many countries, but in the US it is almost the norm.