You have to keep in mind the whole rookie experience. During high school and college the play the season and just some off season training programs. But then they went an entire season in college last year, then played their respective bowl games. From there they immediately go to intense training for the combine. Getting their forty timesdown and working with individual specialist for pro days. they spend the next couple months trying to get all the measurables as great as possible. Then they get drafted and spend time in rookie camp and voluntary work outs, then they are at camp trying as hard as they can to get caught up to speed with the nfl and play books and teammates. Lol then its 4 preason games and 16 regular season games. ITs alot to handle.
Athletes or not, its more than just a few extra games, it's more like 18 months non stop. Thats why rookies tend to burn out a bit before the season ends.