I'm puzzled by contradictory policies in North America regarding rabies vaccination for humans and animals.

My understanding is that:

1. cats are now considered to be vaccinated for life once they've received their initial series of rabies shots, yet

2. people who have been vaccinated against rabies still need a course of further vaccination if exposed to rabies, and

3. need booster shots every six months to five years (depending on their risk of exposure), and OTOH,

4. that no person vaccinated for rabies has ever caught the disease in North America, with or without booster shots or post-exposure vaccination

(the exception to point 4. being some Peace Corps workers who received anti-malarial drugs during the period of their course of vaccination)

This makes no sense to me for several reasons.

a) if cats can maintain immunity throughout their lives, why would human beings need boosters after six to sixty months, and post-exposure vaccinations in addition to that?

b) what does risk of exposure have to do with immunity? If a course of vaccination confers reliable immunity only for six months, then everyone who's at risk of exposure should need to be boosted every six months

c) if the immunity provided were as tenuous as these polices suggest, surely lots of vaccinated people in North America should be getting infected with Rabies

d) my partner and I still have adequate titers a quarter century after our initial course of vaccination, without boosters.

Is there a rational explanation?