Maurice Clarrett did exactly that and was beaten in court by the NFL. But like K2C said, I think 18 year old kids would get killed if they tried to immediately jump to the NFL. Remember, Barry Switzer said the only 2 players he ever saw that could have made the jump from high school to the NFL were Earl Campbell and Marcus Dupree. Another name that was mentioned that could have made the jump was Adrian Peterson. I mean I agree with you WNAS that the NFL shouldn't be able to bar kids like that. But I think most understand that kids would get killed at 18 years old. But you have a point in making kids that do this sign a waiver that they cannot sue or bring any lawsuits if they do get physically impaired. Lol
Also, why would the NFL invest in a football minor league or any other semi-pro farm system as an alternate route for kids who want to skip college, when they already have the perfect setup with how college football is setup today?? The NFL has their cake and eats it too with the current setup.
The NFL may be a dream for the majority of college recruits but even if they could go directly to the pros not many (as in almost zero) would even get a look. So there is no reason to have the current NFL rule disallowing college athletes leaving school after 3 years. It's to the NFL's advantage for these kids to play college ball so teams can evaluate their skills before investing their money. It's all about the money like just about everything in college and professional sports.