Why do pimple like sores form on the buttocks and perineal area?

Hidradenitis. I would speak to your doctor about your general health. Are you diabetic? Are you overweight? People with hidradenitis supurativa get "pimples" in their groin and buttock and it is not related to weight or diabetes. I find topical antibiotics and/or fruit acid treatments help. I would see your family doctor or a dermatologist.