This is an under 20 (i.e. up to and including 19 years old) tournament so the notion of sending current (or former) NFL players is missing the mark a bit.
However, if this tournament was important and prestigious enough, with appropriate future earnings safeguards, so that the top talent made themselves available I suspect the outcome would be different.
For other nations, this will be the best they have - admittedly for young people who have been playing contact football for a couple of years at most and obviously from a relatively small pool of athletes who participate in a niche sport. I think it's as popular and supported as a sport in Japan as any other country outside of the US - so it's not really surprising they field a good team for this competition. Japan is also pretty solid in international rugby, from which football in America originated.
And while you can wallow in the misery of defeat, the alternate view is to be happy that other countries are playing the sport at all and are making a good showing.