Interesting how few succeed despite being superficially similar sports for open field players.
It’s a numbers game.
The USA has a population of 340million and only 32 pro teams.
Europe probably has 500 pro football teams.
The competition for places is huge and you are competing against top class athletes who have played the sport since day one. And come through the very competitive college system.
And it’s actually a very technically different game. In rugby you have to run and tackle for 80mims. In NFL you are either offence or defence, so a very defined role, so may never have to tackle or catch the ball in the whole game. and you only work in 10 second explosive bursts with long rests.