Because that's where diseases attack. Bacteria and viruses. Diseases are like an enemy so tiny and fierce that they're even harder to fight than a giant. Humans found ways of drawing blood to find those enemies.

Humans can survive loss of limbs too, eh. Hell, Shanks lost his arm and he's still a pirate emperor -- one of the four strongest people in the world.

For the amount of blood I've seen you lose frequently, I would say you can survive more blood loss than a human, but treating wounds is about saving as many people as possible. The less who die, the better, eh. To me, that's its own kind of evolution.