Because the NFL is a crazy bird and american football doesn't make sense a lot times. Because of the structure of the game their is an obscenely large impact made by coaching, the quarterback position, and injuries that make it so much fun to follow, especially as a giants' fan who's team have been the epitome of ridiculous things happening on the field, but it also makes everything insanely volatile from year to year. Throw in how prevalent season ending and career altering injuries are that entirely change a team's strengths in an instant. Again my giants are a prime example with JPP and Cruz, two of the premier guys at their positions before they got hurt, now we desperately need to replace them even though we just added a phenomenal kid at Cruz's position. Or the insane beating playing the sport requires, so when athletes start to lose their lust for the game they very quickly fall off hard and the salary stops being insufficient to give the guy that true fire that they needed to be effective.
There's certainly many players who are very good for very long times, but the average NFL career was ~3 seasons, last I checked, and teams have 50+ guys under contract when the season begins, so teams have massive roster-turnover every few years because of the way contracts work and just how many injuries there are. When my giants won 2 superbowls in 4 years more than 2/3rds of the roster had changed between the two teams.
Add to this that NFL teams can not draft players until they are three years removed from their High School graduating class, making draftees all 20+ and much more polished than the 17/18 year olds that the NHL drafts. Plus Football is a sport where supreme athleticism with basic skill beats supreme skill and basic athletic-gifts the vast majority of the time in many positions, so that players don't need nearly as much polish as long as they can understand their schemes and what duties are demanded of them on any given play. Making it easier to cycle through new players.
Then there's the turnover in coaching, with new coordinators comes, new terminology, new plays/concepts/schemes, meaning players are relearning what they're supposed to be doing every 2-3, and that's if they're staying with the same team for a long time. Since NFL contracts are not guaranteed, and performances fluctuate so massively from season to season based on tons of factors, not many players don't get cut sooner or later.
I've rambled through enough of the many reasons why the NFL is just weird and different from the NHL or world soccer. The NBA's just dumb and I hate them cause they're stupid (bitter knicks fan), and I don't know about baseball because that's just too american for me, but the NFL is weird. Hilarious, fascinating, terrifying, and insanely entertaining though it often is, it's a very different game and league.