Fans joke that the NFL stands for "Not For Long" and football is up there with boxing as one of the most brutal (and beloved) sports in America. With the average of NFL career's averaging only 3 years, short even by professional athlete standards, the vast majority of players are spit out of the gauntlet before they hit 25.
Furthermore, the NFL has shifted to specialization; gone are the days of the two-way player and at this point even players who play the same position are categorized into drastically different roles; the NFL enjoys an incredible amount diversity of scheme and play, from scrambling quarterbacks to pocket passers, shifty scatbacks to powerful downhill runners, vertical speedsters to sure handed slot receivers to physical press corners and ballhawking zone defenders, the NFL is all about matchups and that has resulted in the myriad of play styles we see today.