Why do we say "All that glitters is not gold"?

Meaning:

What might seem attractive at first glance doesn't always turn out to be.

Background:

The earliest known record of this expression is in Geoffrey Chaucer's The House of Fame (1384).

Lo, how a woman doth amis,
To love him that unknowen is!
For, by Crist, lo! thus hit fareth;
'Hit is not al gold, that glareth.'

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