Because the injury is temporary and it's a defuses the conflict. One spray and it's over. If you strike someone hard enough to instantly defuse a conflict, they probably have a brain injury.
That shit doesn't happen in the US. The worst we have here are beggars who pretend they're handicapped, or people selling fake shit, but roving gangs of kids who snatch purses and wallets? Never seen 'em. Shootings? We've got plenty of those though.