Because the operate in two different parts of the system. One is about our social systems and the other is about our legal systems.

Governments, contracts, civil and criminal litigation should work on a presumption of innocence (even if the bar is lower in civil court, it still exists), and require evidence (often an abundance of it, or so much it removes all shadows of doubt) to enact binding legal outcomes and punishments.

Groups, events and organizations should work on a basis of empathy and freedom of association. People generally don't want to associate with predators, abusers, and fascists (though the reactions of many commentators in hacker communities often make me question that assumption). And so many social systems are designed to filter those people out. And in more emotional settings like friend and support groups, believing victims is a way to exercise our empathy and provide mental and emotional support and healing for them. Which is the purpose of these groups.

This is like basic civics.

I think this is merely an attempt to highlight the fact that victims of rape have been systematically not believed and probably come from victim support groups. Like many things it's a way to bend the stick in the other direction. I have always translated it as "Take victims seriously" no matter how hard some people online have tried to act scandalized and tell me that it means "Believe blindly what a victim says"

I guess the reasons why the word of victims is generally put in doubt is combination of factors ranging from a lack of empathy from men that identify more with a potentially falsely accused man than a a woman victim of rape to denial because it's uncomfortable especially with happens inside a family (way more common that the rape in a dark parking lot at night trope) where people have ties to the abuser and may even like him.

In many ways. It means taking active steps in making the victims feel welcome and safe in the place they were in. How would you feel if after being mugged by a close friend, others would tell you to let justice work its way (for 6 months) and stay really close with said friend ?

It's about giving support, about supporting the victims and being with them, it's about ensuring that the two of them are separated. Otherwise, not only does the victim lose trust in one friend, they also lose everything else. Nobody is asking you to go at the accused and beat them up. They're asking you to act as a buffer, as a safety net. It means questioning and grilling said accused because, hey, this other friend (that you trust, right?) has accused them of fucked up things. And yes, it means spending less time with the accused. Because you know what ? The rate of false accusations is extremely low.

This is a bigger issue than can be thoughtfully discussed in an Internet comment thread, but if you are interested, I highly recommend Amia Srinivasan's "The Conspiracy Against Men" for a thoughtful, nuanced, and comprehensive examination specifically of "believe women" as a slogan, and more generally of the way our culture navigates accusations of and regulations around rape and sexual assault. It is the first essay in her book "The Right to Sex."