I agree that the consistency of design shows that Native Americans set out with a design in mind. But that doesn't mean that it was necessarily a good design. If you asked a Native American from 400 years ago why they were building a bow the way they were, I imagine a great many of them would shrug and say "that's how I was taught to do it, and it works for me."
Primitive peoples had a lot of other things on the go, and when it came time to hunt for food, most of them were likely more concerned with what had worked for them before, than what might work better for them next time. They didn't always have the luxury of time and energy for experimentation, and they certainly couldn't jump on the internet and have it out about what the best design was. Experimenting with a new design was often gambling with the well-being of your wife and children.
Its foolish to assume that primitive peoples didn't put any thought into it. After all, they were just as intelligent and inquisitive as we are today, and most people are always looking for ways to improve their lot. But neither is it wise to attribute them supreme knowledge. They didn't have the wealth of information that we do, nor the time or energy to spend all day experimenting. They arrived at designs that worked based largely on trial and error and natural selection. That's not putting them down by any means. All of modern science and technology is built on a foundation of trial and error, and more than a little failure and hardship. Every one of us on this site is informed by their success and failure.
I can respect the work they did and the things they learned, while still acknowledging that they may not have been entirely right, or went about about it in the best way.
All we can really say is that Native Americans knew at least one way that worked pretty for well them. Was it the best way? Maybe it was maybe it wasn't. But it worked well enough to support a thriving population for generations.
Think of it this way. If you had to put dinner on the table for your family tomorrow, what bow would you grab off the shelf? Your tried and true shooter? Or an untested design that you thought might work a little better? This kind of mindset is necessary for survival in primitive groups, but it tends to hold back innovation and creativity once a solution has been found, even if that solution is only a marginal one.