With about double the players on a college team over an NFL team the D1 team can afford to speculate.
Also, when you recruit the "Best Player Available” you are recruiting a player you will most probably use. This is because collegiate athletics has a near 100% turnover rate every 5 years, so BPA recruits will get a chance to produce.
In the NFL with limited rosters, when you ignore need and draft the "Best Player Available" you are drafting someone for the bench. You may or may not ever use the drafted player, because the guy ahead of him is acknowledged as good. If the drafted player somehow starts, you drafted a good player to the bench because he sits an already good player. One way or the other you are relegating (drafting) someone to the bench.