That's because these people don't really believe in heresy. Even the SSPX (both Menzingen and the Resistance) talk about "errors" of the Conciliar church, but they don't really believe there's heresy--even when they slip up and use the term.
Heresy has consequences, but most people refuse to believe it when the little boy stands up and truthfully says, "But he has no clothes!" Instead, they just try to shut him up and pretend he is still wearing his royal garments.
If the current teachings of Conciliarism are nothing but "errors", then why on earth did the Church worry about Arius? Of course, the Conciliar sect is already reforming its views of Luther--and guess what--they now say that it was the Church that was wrong! This is just an error? Or is it heresy--that puts one outside the Church?