There are always going to be some people who lie. Those who advocate believing everybody 100 percent, or believing people especially because they are a member of some particular group (a woman, a member of a certain race, etc.) are just as bad as those who don’t ever want to believe an alleged victim/ survivor. Even children, who were believed for a while to never lie about sexual abuse, have not been truthful where they were influenced to lie by a poorly run investigation, see for example the McMartin and similar cases. Society tends to go from one extreme to the other on this instead of just being realistic and looking at every claim and the evidence for it.
The other reason people have trouble believing is that in many of these cases there is either no evidence (no video, no admission, etc. ) or else the accuser had some sort of long-term relationship with the accused, including after the alleged abuse went on. I am well aware of all the reasons why victims would have such a relationship, including to try and normalize or deny the behavior, acting out of fear, acting to preserve their job or perhaps get some benefit out of a bad situation, thinking that no one would believe them so they better just go along with it, etc. but to ordinary people not in the situation, this looks bizarre, and again there are probably a handful of purported victims who are being less than truthful or are being manipulative after the fact in hopes of getting money or some other advantage.