I'm still not sure about the mechanics that lead to rabies being incurable. I know that it can be treated before any symptoms show up, but why is it that once symptoms show the person is a dead man walking?
This is because rabies is a viral infection of nervous tissue that propagates through peripheral nerves into the brain and causes brain tissue inflammation (encephalitis).
As long as the virus is in the brain there is no way to get rid of it. The main trade-off here is that everything that would kill the virus will be as (or even more) aggressive against the brain tissue, and impairment of the latter will lead to really heavy deficits in vital functions like breathing and thermoregulation.
The first manifestations of rabies are those due to brain damage. This means, the virus is already there and the brain is already fatally damaged.
It may actually be curable, as shown by success stories involving the Milwaukee protocol in which the brain is effectively shut down to allow for the immune system to eradicate the virus. In most cases, however, it is fatal once symptomatic. Note that the Milwaukee protocol boasts an imporessive (sarcastic) survival rate of under 15%