Most don’t. That’s the problem.
It’s not a prosecutor’s problem, though. Their job is to maximize the use of their limited resources to be as effective as possible with their limited time. Cases of types which have a lower chance of conviction need to meet a higher bar before moving forward or they’re just wasting their time and letting criminals go free.
The underlying issue is that it is often very difficult to get a conviction for this offence. It does not help, in fact it hinders, if there are elements of 'rape myths' still lingering in the minds of many policemen. That needs to stop. When it comes to prosecutors, it is often too late. Though I think it's fair to say that even if policemen
were rid of those myths and be inclined to take reports as much more likely than not to be true (as it seems they are) it would often
still be hard for them to establish the facts needed to secure a conviction.