[uououuttt]

It's also that American football has a lot of downtime between the relatively short plays, whereas Rugby has a more constant style of play. An entire NFL game consists of roughly 11-12 minutes of actual gameplay -- the most recent numbers for rugby I could find put it at ~35 minutes. So, players are less fatigued and can hit harder and faster.

[untog]

Also many more substitutions, which plays into exactly the idea you mentioned.