They aren't quite that effective, though. And, because sex is a very widespread activity, even a few percentage points of contraception failure can and do result in quite a lot of unplanned pregnancies.
Also, dealing with rape is not that easy. First, it can be extremely traumatic; being able to act entirely rationally within a few days is not at all a given. Secondly, it's frequently associated with a sharp power differential, i.e. the rapist is often your father, your boss, your partner who is also the only source of income, your partner who might hurt your kids, or someone who otherwise has leverage, making reporting more difficult (even if it's the right thing to do).