We college fans like our team's players to be drafted, because it shows how good our favorite team is. Pro teams like drafted players because it locks in their first shot at those players. But what about the players themselves? Those drafted in those last two rounds are likely to get signing bonuses of only $40-50K, vs the typical $4k for an undrafted free agent. None of these players get any other guaranteed money, and receive the same per diem rate in training camp, and the same minimum salary if they make a roster, and same weekly amount if they only make the practice squad. The trade-off for getting a larger (but far from huge signing bonus) is the lack of any "say" in where they get to take their first shot at making a roster. The higher-quality undrafted free agent likely has the potential to choose among several teams, weighing such factors as the "fit" to his skill set and the number of other players on the roster (or drafted) with whom he'd have to compete for playing time.
Is that extra $40K worth giving up some level of control?