Personally, it's because it's not worth my time. Why should I have any sort of dialogue with such people when it's not going to be productive and will provoke a massive amount of vitriol? At best, it's unproductive, and at worst it's career-ruining.
I think that most people think the same way, and as a result there are absolutely no consequences when the activists go from "challenging" to "outright hateful."
This is one reason I am inspired by reading about Nelson Mandela. He fought for both equality and forgiveness of his former oppressors. Once apartheid ended in South Africa he fought against the desire from many of the former oppressed to extract revenge. An early example:
"During my lifetime I have dedicated myself to this struggle of the African people. I have fought against white domination, and I have fought against black domination. I have cherished the ideal of a democratic and free society in which all persons live together in harmony and with equal opportunities. It is an ideal which I hope to live for and to achieve. But if needs be, it is an ideal for which I am prepared to die." ~ Nelson Mandela [https://en.wikipedia.org/wiki/I_Am_Prepared_to_Die]
IMHO, this position is more philosophically consistent than what many circles in American academia espouse. As I recall, Mr Mandela had to fight a lot of over-zealous activists in his own party who wanted to implement punishment or retribution to the white apartheid supporters. Though it cost him greatly in terms of discontent and difficulty but stands to me as an example. But it's been widely acclaimed after the fact.
I think that most people think the same way, and as a result there are absolutely no consequences when the activists go from "challenging" to "outright hateful."