Knives, like just about anything else, reflect culture. But when looking solely at knives, I'd say it's near impossible to glean many specifics. Like anything else, some designs reflect pure aesthetics, some reflect practicality, most probably a combination of the two.
I'd say it's difficult to make a case that US Americans have been particularly adventurous, resourceful or clever in in any particular field. Except for one: Free enterprise. The US has been extremely successful at creating markets for new ideas. This applies to everything, not just knives. One merely has to look at the wide range of knives available to American consumers today. More variety in shape, size, steel etc than ever before. They don't reflect practical needs, but rather, created needs. The market creates objects of desire, fulfilling our needs well beyond the practical. This is perhaps one of the most visible difference between a free market economy, and a centrally controlled one such as the old Soviet system, where the market may have enough goods to fulfill your needs, but that's all. The US has been a leader where markets are free to fulfill not only our needs, but every desire, too.
So I'd say, if some space alien visitor were to view an American knife store, understanding the function of knives, they might deduce that Americans must have an awfully prosperous existence in order to support such a wide array of items that all essentially do the same thing.