They werent.
Lots of old mountain hands kept flintlock and for good reason, well into the caplock age. Drop your tin of caps in the snow and it's ruined. Once the caps are gone the gun won't fire etc.
I suspect once the flintlock became "obsolete" due to evolving technology. that they became cheap to purchase and thus found their way into the hands of many who could afford little else.
Flints could be found anywhere and Knapped into shape to keep the rifle going as long as powder was around. Hunters would even harvest balls from th insides of game and recast them in their mold.
I bet there are less than a dozen people on this forum with the ability to knap their own flints. Most people wouldn't know a flint rock if they tripped over it. I bet most mountain men/ trappers and the like didn't possess the skill either.
I think we need to separate the romance from the reality.