I think the reason is that there are a huge number of supplements marketed that have very little basis in science. (A few small, poorly designed studies published in third-tier European or Asian journals, or studies done on rodents, can make for great marketing, but don't prove anything.) Therefore, the vast majority of supplements are basically ineffective, or depend entirely on the placebo effect. Many people, having wasted money on a few ineffective supplements, will give up.
Even supplements with therapeutic value are often the victim of runaway speculation on their benefits. You mention fish oil in the question. Fish oil was shown to help one condition (in doses larger than most people are likely to take), and soon people were advocating it for every disorder under the sun. For example, I personally saw a huge number of articles and advertisements suggesting it would help Crohn's Disease. Yet, once it was actually properly studied, it had no benefit for people with Crohn's. Many supplements are marketed for very broad areas such as "digestive support," or "immune support," or the like - the various conditions within these areas span such a range that a physiological effect that might help one condition would actually make a different condition worse.
Lastly, some supplements are only helpful for people who have very specific issues, which would have to be diagnosed by a doctor. E.g., you can get DHEA as a supplement, and if you have a chronic inflammatory condition that's led to a DHEA deficiency, it could have a dramatic effect. But taking DHEA when your levels are normal can cause all sorts of problems. But, if you go to see an endocrinologist to see if you have low DHEA levels, what's the likelihood that he or she will tell you to go buy an OTC supplement as opposed to a compounded prescription?
So, the TL;DR version: Many supplements don't work, and the ones that do only work in specific cases, so people get tired of buying things that have no benefit to them.
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