Daryl's right. The early German and American rifles were also VERY well designed and were made to fit, one at a time
In the 1820s in America, the industry was headed for a period of mass production, and the coming era of the percussion rifle doomed the era of the "hand made one at a time" rifle. These things never change.
Look at the stock designs of these rifles. Ignoring the art forms, if we look at any well made rifle we see that the basics are the same. 1750, 1780 1860 ........ we are still Human Beings.

Be it a 1750 Jaeger or a 2000 double rifle, they must fit a human body, and so the foundations don't change much. Only the art forms change.
The market will demand what it demands. You see, just as now, the average man could not afford a hand made rifle, and if a military or quasi-military musket, or a "pattern rifle" is available at 1/4 to 1/3 the price of a hand made rifle, most people will buy the less expensive rifle.

How many owners of Ruger, Remington and Winchesters are there in the USA and Canada today for every owner of a custom "fitted rifle"?
It's not that there was anything functionally wrong with a 'pattern gun' like a Derringer trade rifle, or a Leman trade rifle, just as the Ruger today is a completely functional rifle. But the idea that "one size fits all" is just not realistic. Sure, almost any man can shoot a factory rifle, and for the large majority of hunters today, who get out and fire about 20-60 rounds a year, a Ruger (or any other one you like) represents a remarkable value for what you get compared to what you pay.
But no factory made rifle can ever be expected to have the refinements of one that's hand made.

In the days before "standardization" in the arms industry, ALL arms were hand made. Now that's not to say all were made well, but all were made one at a time, so in those days you could find rifles that fit better then you could in the 1820-1870 period in America.
The English and the Germans never lost the "hand made" idea of their industry, but the Americans did to a large extent. And that would have been predictable too. Only in America was the arms industry driven by a market place of "the common man" In Europe, it was driven my military contracts and by the whims of the very rich.

So the USA gave the world a lot of designs that have changed the face of the industry. Out of the USA came "cheep" lever actions, rolling blocks, falling blocks, semi-autos, revolvers, good semi-auto pistols, pump shotguns, Auto shotguns and so on. The British and the Germans gave the world the basic concept and the perfection of the double, and the bolt rifle, but most other advances came from America, because of the industrialization of the market. That's both blessing and a curse at the same time.
The designs that came from the USA were well received for their advancements in their mechanical forms, but with such industry came the decline of the hand made stock. The mechanical forms took preeminence over the 'human factors' to some extent.
So that's why the "feel" of there hand made guns is unmatched today, just as it's always been.

Sure, an FN-FAL (John Browning design- i.e.. BAR type action--- with the bolt lock turned upside down) is a "better WEAPON than a fine muzzleloader, but to those of us that love the old hand made guns, that's not an issue.
A $350 Mossburg 30-06 is a far better weapon to trust your life to than a $6,000 Hand made German Flint Jaeger, but if you life is not on the line, you pick up the Mossburg and you feel as if you have a $350 rifle in your hands, because you do.
THAT's the difference!

I am effectively a toy maker. I cater to the wants of men and women that are not trusting they lives to my flintlocks, but are buying them as objects of love and investment. As things that they can feel joy in using, and heirloom quality, so their children and grandchildren will have a family treasure.
I have no intention of making a "better weapon for hunting than a Ruger M77.
I have no intention of making a "better" combat weapon than an AR-15, AK47, G-3 or FN-FAL.

It's a different product to serve a different function and wrapped in a different mindset.

Forgive the ramblings of a man on his hilltop in Wyoming this morning. Just thinking and letting the thoughts come out my fingers.

Hope all is well in the world with all those that read these words, and may God guide you
Steve