With relatively rested players lining up against each other on every down the situation is much different from players chasing each other on the field like they do in AFL.

WRs for example have a somewhat similar situation to AFL players since they are mostly being chased by the defense and they don't get nearly as many contact injuries as RBs for example. Not to mention they are protected by (frequently controversial) pass interference rules.

From what I gather, introduction of the helmets back in the day reduced injury rates.