I didn’t challenge him for two reasons. First of all, he explained how he arrived at his conclusion (i.e. the sign in Yeshiva). We can all, therefore, chose to give validity to this source or not. You, on the other hand made a statement of claimed fact without explaining your source. I therefore questioned how you arrived at that conclusion so I could decide weather or not I choose to give veracity to what you said.
Secondly, It is far more logical for someone to have what they believe to be evidence that the problem does exist. If someone has first hand knowledge of many cases of abuse, they might draw the conclusion that the people they know represent an average cross-section of frum society and that the problem is therefore prevalent. On the other hand, what evidence would lead a person to be certain that the problem is rare. Do they assume that they would know of every case that existed in their community?