Who are bystanders and why do they just stand by? Why don't bystanders get involved?

What I think:
I believe that a bystander is a person who witnesses sexual violence/rape, or is present when there is an indication that it may happen. A bystander can be anyone. It could be a stranger, a family member, or a friend. They could be young or old, man or woman. It could be anybody.

What the sources say:
"A witness is someone who has firsthand knowledge about a crime or dramatic event through their senses (e.g. seeing, hearing, smelling, touching), and can help certify important considerations to the crime or event. A witness who has seen the event firsthand is known as an "eye-witness". Witnesses are often called before a court of law to testify in trials." - source

Dialogue between a bystander [B] and a perpetrator [P]:

P: [to girl] Come on babe, lets get back to my place. You've been teasing me all night, showin' yourself off, dancin' so close to me, you owe me a little something something.
B: Hey man, leave her alone.
P: Piss off buddy, this is a private party.
B: Look, she's not interested. Let her go.
P: She's MY girlfriend, and she owes me this. She OWES me, you get what I'm sayin'?
B: She doesn't OWE you anything. If she doesn't want to leave, she doesn't have to. So just let her go.
P: Look, nobody asked you to get involved, so why don't you just get out of our faces and mind your own business?
B: I'll leave when you've let her go. I wouldn't want somebody to do that to someone I care about.
P: Don't make a scene. Just leave and I'll pretend you never stopped us.
B: No, we're not letting you leave.
- other people join B -
B: Just get out of here and leave her.
- P leaves -

Dialogue between a bystander [B] and someone who didn't get involved [S]:

S: Why didn't you stop P last night? He raped me.
B: I'm sorry, you were dating him, I thought you were just playing when you left.
S: Just playing? I was telling him no. He was drunk, you know what he's like when he's drunk. I don't like being around him like that.
B: I'm sorry, I didn't think you'd get raped.
S: Well what did you think was going to happen? He was hurting me before we left.
B: I'm sorry, I guess I just wasn't paying enough attention.
S: Stop saying you're sorry. You just left me to him. You didn't want to get involved. You could've stopped him.
B: It's not my fault, I couldn't have done anything.
S: No, you could have. You just didn't. You just stood there. Everyone always just stands there. That's why shit like this happens.