I've come across a sentence which says
"The CEO of many corporations is responsible to the board that hired him"
notice the bold-faced part. Why can't we use "are" here?
Simply because the CEO is singular.
It would also be fine to say:
the CEOs of the companies are responsible...
It’s clear that each company has only one CEO.
The CEO of many corporations - implies that, a single person is in charge of more than one corporation. If this is the case, then this sentence is fine. Otherwise, both the CEO (CEOs) and the to be verb would in plural (are).