Because rabies is rare, so even though they'll kill animals to test them when practical or when they haven't been under observation for very long, the chance of transmission is low. Most of those 6 gorillion animal bites are from dogs and cats, and in the vast majority of cases they can find out if the animal has been acting funny/locate the animal. Those 100,000 vaccines are from people who were bit by things like raccoons, or by animals who were acting strangely, when the animal couldn't be captured. The reason people who work with animals get vaxed is because they get bit so often that the risk outweighs the reward.