Because I have seen statistics on the "rarity" of various types of wounds. Just because only one wound in 5,000 was the result of a bayonet (because other weapons of longer range accounted for the vast bulk of the wounded) does not mean that hte number of bayonet wounds was insignificant or particularly rare.
That's about one wound in 333 inflicted by a bayonet. Since there were literally millions killed in World War I, there were tens of thousands of bayonet wounds. While to a statistician, 0.3% is insignificant, it is rare only in a general sense.
What most people would consider rare to mean (and what I suspect you read "rare" to mean) is insignificant. Tens of thousands of injuries (the vast majority of which resulted in death) is not insignificant in any absolute sense. It is just statistically smaller than the number of people injured by small caliber arms through artillery.