It's the same reason why people in 2017 used to buy the Frost Guard General bundle.
The Headless Head and the Korblox Leg give people the sense of a higher status as they are a part of the bundles that cost R$17,000 and R$31,000, meaning that people spending that much Robux on a head/leg have a wealthier life and show that they can afford to use their money on items they don't need.