Because they feel embarrassed.
Because they are frightened a retribution from a fucked up system.
Why do you care if women come forward and don't name names? If they did, you would want proof anyway; proof that they likely couldn't provide. If you know any women that work in a professional context, ask them if they've ever endured even a hint of sexual harassment and the answer will likely be north of 90%. Hell, ask any woman period and it will be that high.
For some context, look up the recent comments from Terry Crews, a huge fucking dude that was sexually assaulted/harassed and didn't say anything.