They survive because people want to believe. Look at the history: the phrase “snake oil” exists because people actually sold products by that name as a remedy. There were also tonics based on radium.
The story of how the phrase snake oil came to be is a fascinating one. Clark Stanley, an American, ripped off a Chinese product which was actually a decent anti-inflammatory, and didn't put any snake oil in his snake oil, which is why it didn't work!
“There's a sucker born every minute.”