It comes from long ago, when terra cotta or cast iron pots were used days after days in the fireplace for the daily meal which was often soup. Hopefully stews when people had the chance to have something else than vegetables and potatoes to eat.
Pots use to be left on a tripod or suspended from a cog or a chain in the den of the chimney. Soup was cooked, again and again, everyday in the same pot, using leftovers of the previous soup.
Thus they there was always soup in the pot and it was rarely washed. It was said that those old pots got better as time went on and as food flavors got encrusted in the pot.
This expression is now used to mean that what is done many times, the old methods that have proved their efficiency… are often better than the new ones. And by extensions women of middle age, are said more experienced thus better (don’t complain!!! I’m just explaining the idiom !!!).
Ok. Today we wash our pots but the figurative meaning remains!
I took this photo in an old country house kitchen. Gas, an old pot, a good meal … What else!