The thing is you cannot just switch sports.
When we are talking about the really TOP elite of football those kids get into it at age 5. From that age on every day consists of hours of football. There are scouts looking at prepubescent kids all over the world ready to sign them.
That's the point. Since so many sports are competing for "player attention", people may commit to the "wrong" sport early on, be decent, and then top out at an age where it's too late (in terms of going pro) to switch to a sport they may have been great at.
In a hypothetical world where every kid plays only soccer, every potentially great soccer player has been practicing the sport from an early age. In a world with 10 competing sports, some potentially great soccer players might have be playing baseball or basketball from a young age up into their late teens.