why do people who sell to "cash for gold" outfits not sell their gold to other civilians?

As has been said repeatedly, it’s because only investment grade gold makes any sense even as a risk to take in future commodity pricing. It’s a specialized business to appraise gold and make the contacts to sell the gold to be melted. Jewelers have to go through an excruciating level of training to operate their businesses, so it’s not an entry-level or street corner trade. Bullion is equally far from any consumer skill level to access, assess, store, or trade.

IOW, you have to stop thinking of trading gold and start thinking of trading bullion. That changes everything.