Because they make it through college eating McDonalds, drinking Cokes, Paryting, not stretching, not training properly, not getting enough sleep etc and they think they can continue to do that throughout their NFL careers. When the trainers tell them to start eating responsibly and taking care of their bodies properly they don't listen because they've done the same thing all their lives and it's worked up till now. They don't realize that that they are getting older and they have just raised the bar as far as physical exertion goes and their bodies need to be taken care of or they are continually going to be rehabbing little nagging injuries. It sometimes takes 2 or 3 years for them to figure it out and sometimes they never figure it out.
Powerhouse schools that continuously put guys in the NFL are the exception. Do you think Alabama players have that issue? Hell no. The school has a football budget somewhere around $65 million. They make sure their players have the proper fuel to practice football and monitor what they eat. The foods in the athletic dining hall are color coded:
Green- eat plenty of these (ex. fruits, vegetables, lean meats, eggs and fish)
Yellow- eat in moderation but increase your intake on more intense workout days (ex. pasta, potatoes, rice, cereal)
Red- eat sparingly (ex. fried chicken, ice cream, cookies, and prime rib)
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