Spammers continually send email with different subjects and offers in order to entice users to buy their products. Many of these products are; fakes and knock-offs or potentially dangerous to the purchaser. Due to the low cost of sending spam, even with the extremely low response rate, spammers can make a significant profit fairly easily. The more emails a spammer can send, the greater his profit while the cost remains nearly zero.
Spammers rarely honor opt-out requests. It's work for them to process opt-outs, sending less spam doesn't cost them any less, and who knows, you might be one of the suckers who responds next time.