Because this is not 1964, almost nothing is manufactured now the way it was in 1964, they are not the original manufacturer with the original tooling, and they are making a production run of several hundred rather than several hundred thousand
with all the decisions and compromises those facts entail.
No, it would save you time and money. It would cost the rest of us time and money. Most buyers of replacement gas tanks do not care if the seam crimp is of a slightly different shape, or if a random string of letters and numbers that was present on the original tank where nobody ever cared to look is not present on the replacement. Most buyers of replacement gas tanks have the goal of a non-leaking, clean, correct-fitting gas tank in their car, full stop. You, on the other hand, want all those things and exactly-as-original bends, stampings, markings, finish, and every other detail. Doing a reproduction part to your standard is much, much more costly and time consuming than doing it the way it's generally done. And then we could talk about the enormous licence fees it takes to put trademarked factory markings (such as a Pentastar) on a reproduction part.
Your definition of "factory replacement" is not the same as the one most people use.
There are problems at the other end of the scale, to be sure, where "reproduction" parts are pathetic halfassed copies. But those repro gas tanks are made for the 99.9% of buyers who do not care the way you do. Your way isn't wrong, it's just much less common and much more expensive.
Next question?
