Why do you have to play college football to get drafted into the NFL?

Why is that young athletes in America must play amateur sports for schools in order to become professional athletes? 

Should NCAA academic eligibility determine whether an athlete qualifies to play for a team in the only amateur sport program that develops professional basketball and football players?

I've been asking myself these types of questions for a few years now. Michael Lewis has touched on the apparent contradictions in the NCAA system in bunch of his work over the last few years, most pointedly in "Serfs of the Turf" - http://www.nytimes.com/2007/11/11/opinion/11lewis.html .  I think the United States is the only country on earth that in which an amateur athlete's career development is completely dependent on educational institutions.

Pele played soccer professionally at the age of 15. Wayne Gretzky dropped out of high school when he was 17 years old to play in the WHA.  Neither of them had to play for a school, and neither of them had to meet academic standards in order to qualify to play their sports.  I'm guessing Walter Gretzky might have taken away Wayne's skates for few days if he came home with bad grades on his report card... but no organization or school told Wayne what his minimum GPA had to be if he wanted to make it to the NHL. 

Today, soccer players in Europe, Africa and South America sign professional contracts  as young as 16 years of age.  They don't have attend college classes and play for a college teams in order to become a professional athletes.  In fact schools, and academics have absolutely nothing to do with the how these young athletes pursue their amateur/professional soccer careers. 

Top youth hockey players in Canada are drafted to play in 'Major Junior Hockey' leagues at 16 years of age and play as amateurs until they are 19.  Junior hockey teams have owners, these owners run the teams to make money. Its not a big money business, so you only get involved if you love hockey.  The players aren't paid salaries but they receive world class coaching and get paid enough  for room & board and 4 years worth of college tuition (assuming they want to attend school, if they choose to simply play hockey that's ok too).

The MLB farm systems exist as a functional alternative to NCAA for baseball players.  They can become a pro baseball player without attending school.  But football and basketball players must attend a college to progress their athletic careers.  The choice is go to college or end your athletic career.  The NCAA seemingly has exclusive control of amateur player development for these sports.