I know this was a rhetorical question, but here’s a real answer anyways.

Evolution isn’t smart, and it doesn’t care if something kills you, so long as you can pass the genes on first. Boobs are for feeding babies. If more mammary tissue mean you can make more milk for your baby, you’ve got a better chance of your child surviving infancy and passing that gene on.

Also, boobs are a large percent fat, which functions as both nutritional and hydration reserves that are used during pregnancy/breastfeeding to ensure the baby survives and passes the gene on.

For better or for worse, my back pain doesn’t make me any less sexy.