re: Players "declaring" for the NFL who will not be drafted.

It's all about getting a head start to getting the 2nd contract. If he's going to be an UDFA, it's doubtful that coming back will propel him to a draft spot that would make it worth coming back which would push his 2nd contract back a year as well. The rookie salary cap has led to many players declaring early regardless of draft stock since it was put in place.