I think you're confusing "proof" with "reason to believe". We have plenty of reason to believe they're telling the truth. The similarities between their stories, for one. The fact that actresses who played the Cosby kids' friends had their own stories about creeper Cosby, for another. Perhaps more significantly, the fact that these women have absolutely nothing to gain by lying about this. Heck, just them talking about this has already made them bad guys in your mind and the minds of many others. They're probably going to come out from this worse off than Cosby, since they don't have millions of dollars and publicists to shield them.
I hate to bring up Occam again, but do you really believe it's more likely that all of these women are lying about something they stand nothing to gain from, or that a rich and powerful guy abused his power and money to do unscrupulous things to get his dick wet?