The answer is that Hashem created man with free will, free will to believe or not to believe, to accept or not to accept. The capacity for free will was included this most amazing ability to accept the most farfetched, ludicrous positions as long as it fits into a person’s agenda.
This seems to be the answer for Rashi. The Torah is teaching us that the human may be brilliant and wise, but he has a weakness. His pure judgment can be easily influenced. If a man accepts a bribe, it will color his vision. He will lose his detached sense of judgment and will no longer be able to rule objectively. He may think he won’t be influenced, but it is human nature to be pulled, and he will no longer be capable of objective clarity. He will be blinded.