Blood does replenish but it takes a while. It takes about 24 hours to replace the volume of a pint of blood and 4-6 weeks to replace a pints worth of red blood cells. If you've suffered major blood loss, you need blood NOW or you're going to die in minutes.

When you donate blood, you only donate what you can spare. The average adult can spare 1 pint of blood to donate but that's very different than getting shot or stabbed and losing 5 pints of blood.