Of course, if the victim claims the crime didn't happen, it obviously makes it harder to prove it did, and the prosecutor will take that into account when deciding if the case should be brought to trial.
But there are important reasons why one wouldn't press charges despite having suffered the crime; a few of the obvious ones include fear of retaliation, being unwilling to be in the presence of the perpetrator(s), lying to protect them, etc. And despite that refusal, there still may there be enough evidence to prove it happened.