Partially to ensure consistent protection (Ab levels fall off before they go away), partially because we don't check our pets for seroconversion, and partially because they (hopefully) aren't constantly exposed to it. Plus, reliably effective post-symptomatic therapies still aren't really a thing.

When I worked in Rabies testing we had to have periodic bloodwork to ensure we were still immune. Of course, we had so many positive specimens each week that we never had to worry about it...