Bear spray actually has a much LOWER concentration of OC than products designed to be used on humans. Maybe bears are just sissies.
Bears kill roughly 1-3 people a year in North America each year. Are most bear attacks "attacks" or posturing and attempts to drive us from their territory? Go take a quick gander at the bear spray studies. Note the success rates. Note the relatively small instances of use on charging bears vs posturing/curious bears. I think you'll see some correlation. You could, perhaps, draw some parallels to how dedicated your human/canine attacker are and note that humans and the animals we socialize are not equivalent to wild animals in many regards. Why do we euthanize bears that get too accustomed to human contact?
I'm certainly no bear expert, but it seems like perhaps they are not a good analog for this conversation.