Why do you find shark teeth in freshwater springs?

The simplest answer is that Florida used to be completely covered with water which isn't surprising given how low the elevation is state wide. Since sharks have existed in and around FL for over 400 million years and lose their teeth so often, it's no surprise there are so many to be found. Their teeth can also be found in other places on land but the springs are clear and with the flow of the water it allows for fairly minimal sediment build up leaving the fossils and teeth fairly easy to find.