Football players can be subjected to hits up to 200 Gs when hit, but wearing a helmet can prevent the damage from happening, or at the very least lessen the impact it has on one's life.
Another reason for high pay is the short career of an athlete. The average American will retire between the ages of 65-75, but a professional athlete will last a lot less in their career than someone who sits at a desk all day.
Being put through the kind of physical stress a pro athlete goes through will almost certainly affect you later in life, especially those in full-contact sports.
Knee, back and head injuries can be the hardest to deal with later in one's life, due to the fact that they are parts of the body we use every day for many different activities.