Because people don't always swim in places where they frequent. They aren't known to always be close to shore near surfers etc, but when they do, it can have disastrous consequences. People know the risks I'm sure, but the "It won't happen to me" thoughts override that!
As for people who swim in areas where there's always great white sharks around - then they're just stupid.
And it's the Bull Sharks you want to be more careful of - they come closer to shore and I think round the coast of Hawaii, they've attacked more people than all other types of shark together (I did research since I went to Hawaii!)