Rugby isn't more athletic it's just more brutal.
How so?
The lack of pads and rules for one. They do the same amount as a football player. Get the ball, run, jump on/over people.
Rugby players do a lot more of the running and jumping, but NFL guys hit hard. I don't know if you realize it, but two guys weighing 200+ pounds sprinting into each other is not a nice collision. Yes they do that in rugby, but each person slows down a little you can bet.
@proleg, we play rugby here. My schools rugby team went really far this year too.
this is a rugby tackle.

this is a football tackle

To make it fair, that is a college football team, and an international rugby team.