I assume this is where you got your info from... measuring how many trips to the emergency room each accounted for.
  1. Basketball: 512,213
  2. Bicycling: 485,669
  3. Football: 418,260
  4. Soccer: 174,686
  5. Baseball: 155,898
  6. Skateboards: 112,544
  7. Trampolines: 108,029
  8. Softball: 106,884
  9. Swimming/Diving: 82,354
  10. Horseback riding: 73,576
  11. Weightlifting: 65,716
  12. Volleyball: 52,091
  13. Golf: 47,360
  14. Roller skating: 35,003
  15. Wrestling: 33,734
Ok so which of these activities did more people participate in: swimming or boxing? Roller skating or boxing? Baseball or boxing? Riding a bicycle or boxing? ECT ECT

And again it comes down to how serious an injury is, and I content that an injury to your brain, which is irreversible, is more serious than an injury to a limb, which heals in time.