The “cure” for rabies is to treat it with a vaccine prior to symptoms appearing. The rabbies vaccine is 100% effective and you will not become symptomatic if you treat soon after the bite. The Milwaukee protocol has been tried and it’s a last ditch effort for people who didn’t get the vaccine shortly after the bite and are now showing symptoms. They don’t even know if the Milwaukee Protocol is what prevented death or if the people it worked on were somehow resistant to rabies.
Because unless you’re living and working in a high risk environment, there’s no need for a human to go get a rabies vaccine because they can just avoid mammals that are acting strangely. It’s not like it’s airborne, you have to get a penetrating bite from a symptomatic animal to get it, so when that happens you just go to the doctor. You’d still likely get the vaccine even after a bite even if you had been previously vaccinated.
Vets and people who work in animal shelters often get the rabies vaccines beforehand. But even if you have been vaccinated previously, you still have to get it again if you are bitten.
The efficacy of vaccines usually declines over time after administration. The immune system starts to “forget” how to fight a pathogen it doesn’t encounter. It doesn’t completely forget, but it puts the treatment data way back in the archives. So when it encounters the real deal, it can take quite a while to boot up production of antibodies. It also varies by the type of disease.
This is fine for some slow diseases ( which is why sometimes a single vaccination can suffice ), but can be risky if the disease progresses faster than the immune system can ramp up the defenses.
Administering the vaccine as soon as possible after suspected exposure to deadly or highly contagious diseases simply helps the immune system to get the necessary blueprints to get in the fight quicker.
Administering the vaccine before any exposure at regular, long intervals is done to decrease the baseline risk. Sometimes you don’t know you have been infected. Many diseases are not only transmitted by dramatic, obvious vectors. In those cases, it’s definitely better to have some old defense than none at all.
In addition to what Senshi said, if you have recieved the full course of vaccines (4-5 doses spread over a month), any future bites need only 1-3 doses. Also the time within which you have to take the first dose increases from 24 hours to 2-3 days, which can be quite useful to vets in remote places.