Think about all the similar guys who probably did questionable shit - the Rat Pack in general, Hefner, Jack Nicholson, etc. They're all beloved, and even the negative stuff we know about them hasn't really made a dent. Cosby took things to another level by becoming a faux academic concerned about children, then he became America's TV dad; arguably the most beloved TV dad since Andy Griffith. At one point he was the biggest star on television, and that only added to his power.
Remember last year when stories started breaking about how weird he acted behind the scenes on Letterman and other shows? I'd bet that type of shit has been happening for decades but no one said shit because he was the man.