Many rapists could easily wrestle a knife from a woman, but were your trend to take hold in the least, we could expect rapists to regularly carry knives themselves—and thus maintain the advantage.
There’s a problem with that situation as far as rape psychology goes. If a rapist is armed, it is on the assumption that his victim is not armed, because the most important element in rape is both control and dominance.
The same logic applies to crime like armed robbery. Knowing that their victim might be armed does not impel them to still rob, but with a bigger gun.
Many rapists are already so nervous that they cannot actually rape, and have to content themselves with either beating then leaving the victim, or just leaving. By adding the element that their victim *might* be armed radically increases the “degree of difficulty”.
In either rape or robbery, the intent is to rape or rob, not to get into a dangerous fight.