It is because to the harasser they are killjoys. To the aggrieved they are robbing them of agency to help themselves. However, fuck the harassers, and sometimes people need help and don’t know how to ask. I would rather be called a white knight than a creep or someone who did nothing and was thus complicit in the harassment. I’m old and grumpy though and if I’m “helping” it is because I’m sick of the bullshit not because I care if people think I’m a good person; I think that intent matters.