Einstein has been attributed with saying most people only use 10% of their brain which was the context in which I first heard it. It may have been a misquote but the attribution persists and still crops up. I had the impression his remark, (real or media hyped) helped embed the idea in the public psyche.
In any case I always took it to be a figure of speech, not assertion of physical fact.
Edited by Ken Fabos, : No reason given.