For the same reason that massive dudes with cauliflower ears, broken noses, and calloused knuckles are rarely the victims of these supposed "psychos", they're cowards.
Those who are mentally well, smart and diligent enough, to know how to effectively locate and kill a CEO are usually also mentally well and smart enough to know better.
Those with the motive rarely have the means, and those with the means rarely have the motive.