This is a tough issue. If anyone, male or female, were to go to the police and report a holdup/robbery that allegedly took place a loooong time ago, and there were no witnesses nor any other evidence, it would very likely not be successfully prosecuted. I am no lawyer, but that's how I'd expect it to play out. Sad, perhaps, but true.
In the case of rape, the personal suffering of the alleged victim cannot help but come into play as a prosecutor considers the alternatives. But again, without witnesses or other evidence it should be tough to make a case against the alleged rapist. Sad, but true.
In a case like Weinstein's the "fact" of multiple alleged victims certainly would play on a jury's emotions, and that alone could encourage a prosecutor to pursue a case. But all it would take to defeat the case would be one sympathetic juror with the guts to say "no witness, no evidence, not guilty!"
The fact that it's likely he did assault one or more ladies isn't proof that he actually did. Tough case. JMHO