Is there evidence that it is something which occurs often enough to merit the wearing of PPE, or is this one of those just in case a large mammal uses me as a hurdle whilst I'm taking part in a triathlon situations?
It's an accident, one of many caused by different things.....
Live in the countryside long enough and see how many pheasants, deer, rabbits, pigeons, badgers try to commit suicide in your path.
No it's not an everyday occurrence, but it can and does happen.
Bike helmets are not classed as PPE anyway, you know that.