Dopamine neurons in the brain are essential for normal functioning. Parkinson's develops when dopamine neurons die off with age or due to other injury (toxins, trauma, etc.). It takes only a 15% loss of these irreplaceable cells for the condition to begin to show up.
In contrast to the cortex of the brain with its 100's of millions of neurons, we are born with only 250K dopamine cells which are never regenerated.
That's a narrow margin to rely on, we skate on very thin ice throughout our lives. Nature is the staunchest conservative and doesn't throw away the things that work. Our bodies still retain the archaic mechanisms of the earliest multicellular organisms in our brains and bodies.
Looks like blood forming tissues are similarly following Nature's way.