Because what he did isn't sex - it's violence.
And it's violence of the most extreme, sadistic and immoral sort - the rape of boys.
Challenging someone who has already crossed lines he KNOWS he can never be forgiven for, means challenging someone who will literally do ANYTHING to protect himself, because he's got nothing to lose.
So taking him on is taking on a feral, ravening, demonic beast that scared the hell out of people, because someone that openly evil is rare to come by, and personally threatening.
But civilization is DEFINED by whether creatures like him are left unchallenged - or not.
That's why he needs to be destroyed.
That’s exactly why that janitor was so shaken that his coworkers thought he was having a heart attack.
And then Sandusky drove his car back and forth that night until 2 am.
Makes me wonder what he did to intimidate the higher ups.