People purchase a myth and a luxury item.
https://www.scientificamerican.com/article/the-hard-truth-about-the-rhino-horn-aphrodisiac-market/

https://www.theatlantic.com/business/archive/2013/05/why-does-a-rhino-horn-cost-300-000-because-vietnam-thinks-it-cures-cancer-and-hangovers/275881/ ”Some conservation groups, however, don’t think rhino horn’s newfound popularity in Vietnam has much to do with the cancer cure-all rumor (pdf, p.2). The more likely reason, they say, is that the horn powder is increasingly seen as a cocaine-like party drug, virility enhancer and luxury item–”the alcoholic drink of millionaires,” as a Vietnamese news site called it.”

https://news.nationalgeographic.com/2017/08/wildlife-watch-rhino-horn-south-africa-auction/ ”(Rhino horns are made of keratin, the same protein found in human hair and fingernails, and there’s little evidence that they have curative value for anything).