Because of all the documented cases where it has not stopped a person from fighting multiple cops, let alone pressing the attack on a lone female.
Because if it doesn't stop the guy(s), it will really annoy him (them).
Because if the person turns out to have a gun, too, one has brought spray to a gunfight.
Spray is good to have in addition to a gun, for handling lesser threats. It is not a good replacement for a gun.
Edit: A potential example, Pond... I am a trained grappler (wrestling, jujutsu, and aikido). I have been exposed to CS spray in military training. Miserable, and it seriously impairs vision. But, if I get a hand on you, I don't have to see you. I know where you are, and now I am that much more motivated to inflict some damage.