The reason the NFL draft feels like a yearlong event, why we care for months about one rookie added to a 53-man roster, is that there's so little else to help our teams. Big moves are so rare. For the most part, you draft players or sign them as undrafted free agents, and you win or lose as those players survive or fade.
Think about it this way: If the Super Bowl-champion Green Bay Packers were a team in any other pro sport, you'd likely look back on their championship year and discuss how they developed Aaron Rodgers, traded for a pass-rusher, then signed that key safety on the free-agent market. In reality, Green Bay's top two quarterbacks, Rodgers and Matt Flynn, were drafted by Green Bay. So were the five main pass-catchers, from the young Jermichael Finley to the old Donald Driver; same with the top two running backs and most of the blocking. Green Bay was also the first team for the guys who led the team in tackles -- A.J. Hawk and Desmond Bishop -- just as it was for the guys getting sacks, like Clay Matthews, B.J. Raji, Cullen Jenkins and Frank Zombo. Basically, you have an entire championship team, and only one major player, Charles Woodson, is an import. And Woodson is now in his sixth year in Green Bay, a football Methuselah at age 35.
That's because the greatest scarcity in the NFL is health, so the league runs on a form of lemon economics. Lemon economics is a simple concept, really -- it says that if an owner is willing to sell anything that has a short peak life span, it loses value because that very willingness calls the item being sold into question. It's quite literally the used car reality. When you try to sell what looks like a perfectly good car, the first response isn't excitement that the car is available, it's "What's wrong with it?" Same thing in football. If you are willing to trade a great football player something that, like a car, has a brief peak, people wonder. You'd be willing to trade Matthew Stafford? Wait what the hell is wrong with him? It's the shoulder, right?
It's why NFL trades of consequence are rare, and even free agent signings of still-great talent are equally rare. Of trades, there are just a few kinds. Some are: