There's always been rumours. But I can definitely understand why some people thought they were bullshit, like a lot of other celebrity gossip.
The 3 most widely spread Weinstein rumours involved Blake Lively, Jennifer Lawrence and Gretchen Mol. Yet all 3 say nothing ever happened to them, and I believe them.
The rumours about Blake, which started almost a decade ago, implied she slept with Harvey for film roles, and that he was singlehandedly propping up her career and giving her work. Except she's actually never worked with him. He never cast her in anything. People made up whole stories about which films she did what for, and they never even checked if said films had anything to do with Weinstein.
The same thing with Jennifer Lawrence. She already had one Oscar nomination and lead roles in two franchises (X-Men and Hunger Games) before she worked with him on one film. Yet there's no shortage of people who claim he started her career. She hasn't worked with him since that one film, but people were still constantly claiming he was casting her in his films and campaigning for her during awards season, even though the films she was in had absolutely nothing to do with his production company.
Then there's Gretchen Mol. The rumours about her were treated like absolute fact, but they all stem from one disgustingly misogynistic piece of gossip that didn't even name anyone involved.
If those 3 women heard rumours about Weinstein, why on earth would they believe them when they knew the very widespread rumours about themselves to be completely untrue? Their natural assumption would be that other actresses reputations were being smeared like theirs were.
Pretty much every actress under 40 who ever worked with Weinstein or took a photograph with Weinstein had rumours spread about their relationship to him. Some actresses who never worked with him and were never even seen with him in any capacity were still rumoured to be sleeping with him, because 'she's one of Harvey's Girls' became shorthand for saying you think an actress is overrated and untalented.
I actually wonder if he didn't put a lot of false rumours out there himself. That way for every person who heard he was sleeping with or assaulting women, there'd be someone else who could say 'well, they said that about me and it was total bullshit'. Creates a real environment of doubt where only the victims can really be sure of the truth, and the victims obviously won't have gone around telling every other celebrity their personal stories.