Except basketball , baseball , soccer , rugby cricket etc all have dozens of countries playing the sport and a talent pool of 1000's to the tens of thousands as opposed to like the 500 professional and 500 semi professional athletes we have. And if you were to narrow it down to a competitive 23-29yr old prime age bracket we probably only have around 200 as a total pool. That is minuscule as a talent pool when compared to something like basketball and one of like a 1000 reasons why a full on free agency wouldn't work well for our sport.
Not too mention that in those sports
A) they rarely rarely develop the players. colleges and foreign leagues develop like 90% of their talent pools. Can we do that with football?
B) in every other team sport the expected career of an average good player can be 13-18 seasons. Football is more like 8-12 seasons.