Because others haven't been dumb enough to keep sticking their head into the nooses.
You mean like Bill Clinton, who dodged the draft, protested his own nation during the Vietnam War, toured Russia and the Nations behind the Iron curtain while staying with communists, used State police officers to help him sneak sluts and whores into the State House, Lied about smoking weed, used the power of his office to intimidate women into having sex with him, became sexually involved with an employee in the White House and then lied about it under Oath, Sold Secrets to China in exchange for campaign contributions, closed down American anthracite coal production to the benefit of the Chinese, ordered a deliberate coverup of the shooting down of Flight 800, and initiated the US Policy (Housing loans for poor people who couldn't pay their loans) and any number of other scandals and abuses of power to numerous to remember and mention?
Like that?
No, I don't think the problem is about President's sticking their heads in nooses, I think the problem is that we have a system where the Media refuses to cover horrible scandals that affect their political party, and that they make up fake political scandals to use against the party they hate.