There’s a good pop psychology book about this:

“Made to Stick” by Chip and Dan Heath

The phenomenon has something in common with urban myths.

My favourite examples are the Bernoulli effect explanation of how sailing boats work and how aircraft wings fly. Everyone believes this but almost no one understands it. There’s something about the complexity and sciency aspects that make the explanation attractive even though it’s useless to help people understand it. This flawed explanation involves pressure differences owing to differential velocity of air flowing around an aerofoil shape. Only problem being that it doesn’t easily explain how an aircraft wing works to allow an aircraft to fly upside down, plus smoke pulses in wind tunnels prove that there is actually no difference in air velocity across the different surfaces – hence no significant air pressure difference. Nevertheless it was the preferred explanation that appeared in most physics texts. And so gets repeated by the older generation to kids even today.

A more harmful myth was in the news today: that there never has been scientific evidence supporting the idea that salt is bad for people. If anything there are people whose health has been harmed by a mistaken belief that they need to minimise their sodium intake.

http://www.stuff.co.nz/life-style/food-wine/5557009/Salt-facts-not-rock-solid

Apparently scientists remain unconvinced because they want proof that something everyone believes is true cannot be true. And to prove that eating salt is not harmful requires feeding people salt - which would be an unethical experiment because of the consensus medical belief that salt is toxic.