In the case of the Audiomods arms, the designer/master machinist (he’s not "just some guy making Rega-based tonearms in a workshop". For one thing, his arms are NOT "Rega-based", they are entirely his own design and hand-made manufacture, and quite unlike the Regas. Yes, he buys the arm tube he uses from Rega, but that, and that alone, is the sole commonality between his and their arms) sells directly to the consumer. If he had a dealer network, the price of his arms would automatically be double what they are.
The Jelco’s are made in massive quantities, which allows for economy of scale. The more you make of anything, the less each costs.
And then there is the matter of diminishing returns. After a "certain" level of performance is reached (in all products), further improvements cost more per increment than below that level. Experienced audiophiles are searching for the "sweet spot" in the price/performance ratio continuum. Flush audiophiles are willing and able to pay for increasingly small and expensive improvements; good for them! The rest of us have to find products that reside in our own personally-chosen "sweet spot".