It weighs about the same as brass bullets which is what bullets are commonly made of anyways but it's harder than brass.
Another thing about bullets is, when they hit something they don't stay the same shape. Softer bullets hit a body, then they mushroom or fragment. That does a lot more damage than if it were to go straight through. Well normally, at least.
Nobody is using plastic bullets, and nobody is going to be plastic bullets any time soon.