The tactic of using majority opinion to support a claim is called argumentum ad populum.
Here are some examples.
- "There are N-billion Christians (Muslims, Jews ) in the world. So Christianity (Islam, Judaism) is the one true religion."
- The Discovery Institute maintains a list of people who doubt the theory of evolution.
- "millions of people value chiropractic enough to be more than willing to pay for their care with their own hard-earned money" (snipped from here)
Many people find such evidence compelling. Strictly speaking, it's not evidence at all. Evidence should be objective. Opinion and belief is subjective.
But why would fallacious ideas be present in a society? Let's start out by acknowledging that there are LOTS of things we just don't know. Our ancestors can be excused for believing the earth was flat because they hadn't traveled far enough to observe that it was round.
Lack of knowledge is only a starting point. The question still remains, "Why do whole groups of people agree on things that aren't necessarily true?"
Ideas are like viruses, spreading from mind to mind, being passed along selectively; good ideas get passed on, while bad ideas don't... they go extinct. So what makes an idea "good"? More to the point, what makes an idea get passed on?
The spreading and mixing of ideas is called memetics, a cultural analog to the biochemical genetics. A meme gets passed on and mutated from mind to mind, similar to how a gene gets passed on and mutated from generation to generation.
Genetic evolution has no concept of a goal or progress, but rather plays out with mechanical indifference. Whatever gene works best at promoting itself becomes the new popular standard. Ideas are the same. There is no innate necessity that an idea be TRUE in order to be popular. As it turns out, the truthfulness of an idea is only one of the criteria. Other factors play a prominent role (ease of tranfer, humour, topic interest, consistency with other ideas).
In the realm of health fads, a mathematical model of how a medical treatment spreads shows that "the most efficacious treatments are not necessarily those most likely to spread".
That's not to say that majority opinion is useless. Most people value truth. But popularity should never be the strongest piece of evidence forwarded to support a scientific claim.