Sustainability over the long term is not considered. There are painful things in nature. When most of them are eaten, as must happen for something that reproduces so much, each death has some squawking and pain involved. Lots of predators start eating before the animal is completely dead. Killing is something they do almost incidentally, to make the thing stop struggling so they can eat it.
Birth can be an agonising process, lots of cries of pain.
Pain is part of life. We have to have some mental calluses to pain, just as we must have calluses on our hands, if we expect to get anything done. But if my hands are so callused, or I put gloves on, that I cannot feel, then there are things I cannot do that also need to be done. I can't grab things with sufficient precision. Likewise with the mind.