He's not making gold, he's making useful things he sells for gold. Fabricate turns an equivalent value of raw material into a crafted object, so to make 50 lbs of gold bars you'd need 50lbs of gold, but to make a piece of furniture you'd just need part of a tree and 10 minutes.

Just as a note, there's actually too much gold that exists in the Forgotten Realms. IRL humans have only mined 187,000 metric tons of gold, and 2/3 of that was mined since 1950. Without getting into the nitty gritty too much, let's just say there's not nearly enough gold for every monster to have even a small collection of coins, much less every adult dragon falling asleep on a golden pile while everyone else works just fine on a gold standard or something like it.

Is it the dwarven kingdoms out mining humanity? Is gold simply more common or perhaps available in easier, more concentrated ores?

Or are wizards adding more gold from thin air, and dragons provide a natural deflationary pressure by being obsessed with making great hoards of coins?