He's the highest pick, other than McGeary, who hasn't signed.
I found this at BaseballAmerica,com about him:
First baseman Sawyer Carroll has the best approach and swing of Kentucky's draft-eligible position players. His defensive skills are passable, though his arm is below-average. He lacks strength, which shows in all phases of his game. He's primarily a singles and doubles hitter who makes enough contact to draw consideration on the first day of the draft.
He's 21 so he's not just coming out of high school.
Maybe he isn't planning on playing pro baseball is the only reason I can think why an 18th rounder with those "strengths" wouldn't have signed already.
...and why did his parents name him Swain Sawyer Carroll???