Thats quite worrying that you aren't trolling about the american Übermensch.
Well yeah, obviously rugby players are far behind in technique for a sport they will have never played in their lives. Rugby players would be fine athletically, at certain positions, but they wouldnt have the ability to understand the plays, read blocks and coverage assingments, have the correct pad level etc. Even wearing helmet and pads is a lot different. NFL players would be just as deficient with rugby skills and technique.
They dont try because it doesnt interest the vast majority of them. It is not part of their culture, they didn't grow up playing or watching and it was never their dream to play there. I doubt the thought ever enters their minds. Although, to be fair, with everything increasingly globalised nowadays a few guys have expressed interest; certainly nothing that has been particular common in the past. The only rugby (league) player I can think of that did is Jarryd Hayne, and clearly he looked fine athletically...but what do you know, he just didn't quite have some of the technique + tactical knowledge required. If he was younger and took the time to learn, or if he grew up in america on the other hand...
And there is another NRL junior that got drafted in the NFL this year, but he was an academy player and was unlikely to ever make any impact in the NRL.