"Offshore" sheet metal fabrication took off in the '80s when someone in Taiwan realized you could copy the fender from a Ford pickup, punch it out on a surplus or scrapped out press, and ship it back to the US for a fraction of the cost of the part from the OE.
When big insurance companies got behind these parts "Like Kind and Quality" the success of offshore metal fab technology was assured. There is much more to the story, but the bottom line is that technology is out there to get "close enough." Likewise, a few enterprises decided that old car parts have a market, too, and zeroed in on opportunity products that no one else would find profit in reproducing.
For a really high demand part, an OE might rejoin the fray - Ford still produces sheet metal for 'heritage' product such as T-Bird, Mustang, F-Series and has retooled some press sets to bring up the quality. GM has a very active licensing program and works with the aftermarket to produce "OER" which is better than nothing but in many cases, still not production quality steel.
If you ever get the opportunity, it's worth it to tour a metal fab facility - the press lines are fearsome and fascinating, blanks go in one end, finished parts out the other. The bigger the part, the slower they move, but it will sure get your attention.
And it will tell you why it costs what it costs to get OE.