A senior now has the option to return to school. It’s leverage for a top ten selection (where slot money exists).
On the other hand with the draft now in mid July the minor league season is almost over. It means with the exception of maybe a week or two, assuming the player signs immediately the player who returns for a fourth year is going to be already twenty-three when he starts his pro career in Low A. A player who stays for five years of college ball will be twenty-four.
With only twenty rounds of the draft the players selected are more likely to be legitimate pro prospects. We’re in an era where there aren’t a lot of mistakes made in evaluating prospects. 84% of American MLBers come from the first ten rounds. Another 10% come from the second ten rounds. After the twentieth round players are mostly considered long shots. It’s one of the reasons the draft was cut in half.