Argumentum ad populum, the fallacy that if most people agree, they must be correct.
We're tribal animals. Going against the group is usually a bad survival tactic - but just because you echo the beliefs of those around you doesn't mean they're right. Very few of history's great philosophers held popular beliefs, yet we still remember their names. Nobody remembers the conformists.
"Millions of flies can't be wrong!"
The flies aren't wrong. Eating shit is good FOR THEM but it would not be good for humans
Speak for yourself
That raises the question of if you're another fly or another human.
So say we all!
Consensus is the instinctive collective response of the mind to the world whereas truth is its regimented output. This is most obvious in the evolution of language, the truth in this instance is things like definitions, syntax and grammar but this is all after the event and looking back explanation. The real growth of language is from consensus. The definition of the word gay in original form by etymology was carefree and happy which is it's truth but by consensus of usage it was altered to mean homosexuality, which is an entirely separate definition. The consensus idea very easily overran that of truth and became what is real.
I mean, that's just common sense
