People believe in essential oils for the same reason they believe in horoscopes, Myers-Briggs, enneagrams, MAGA, anti-vaxxing, Area 51, and all manner of other nonsense: because it’s a simple idea.
The human brain is evolved to do as little thinking as possible because cognitive activity burns a lot of blood glucose, and for most of our evolutionary history calories were scarce and unpredictable. Those calories could be needed for running from predators or searching for food. Hence our brains are hardwired to grab onto the simple and shun the complex.
The key to successful marketing is to present a simplistic message (X cures cancer!) and avoid complexity (there are actually hundreds of different kinds of cancer with a wide range of genetic and environmental causes and treatments are even today very rudimentary and require significant sophistication in both diagnosis and regimen design in order to have any efficacy…). Selling essential oils is no different to selling snake oil or crystals that align your chakras and help your third eye to focus. Keep it simple and the money will roll in because a great many people are too ignorant and too intellectually indolent to be able to understand they’re being taken for yet another ride.