Stated by a famous guy once, supported only by his personal opinion, and "god given fact" ever since...
That's about as good as, "Because Jesus said so in the Bible!"
Like anything else it's accepted because it makes sense to people and wouldn't have been if it didn't. There is no vast conspiracy there. A billion idioms die a painful death every year but you never hear about them.
We both know that common wisdom is tested by science for good reason: it often proves common wisdom and "making sense" wrong.
The massive cultural bias of the time made an accurate assessment nearly impossible and things have not much changed in the US.
As an argument it's an appeal to an authority that was stating nothing more than opinion.
All I'm pointing out is it's a shitty argument with nothing of substance to back it up. People say it out of habit, but it doesn't lend it any more weight or authority than mere tradition grants to anything else.