That’s an interesting question and you’d have to ask his employers, but the likely answer is “because there’s no point in not doing so.” If he was left in Rookie or short-season A ball, you’d have a 25-year-old guy basically blowing children out of the batter’s box with AA stuff. He’d likely beat up the Rookie League now, but there’s nothing to be gained from that; it would not help him develop and would not help the 19-year-old whose spot he occupies. You have to move them up at some point or else they stop developing and they occupy a spot a younger player should. Alaniz, at this point, is at an age where either you’re going to make the big leagues really soon or you probably never will. If he can be reasonably good in Toledo maybe he could make the big leagues, and if he can’t do it then he won’t, but being good in single-A won’t prove anything - you’d just have to move him to Toledo anyway to see what happens.
Not in baseball, no. Major league baseball is too difficult to be mastered in college. Even star college players usually play a few years in the minor leagues.
The percentage of baseball players who learn their trade in the minors is not 100% but is essentially 99.9%. Baseball players who went directly from high school or college to the majors are so rare that they are the answers to trivia questions. I can, offhand, think of only six examples in my entire 44-year lifetime; Dave Winfield, John Olerud, Jim Abbott, Mike Morgan, Mike Leake, and Pete Incaviglia. There have been a few others I’m sure that I forgot (I’m not counting ballplayers who came from other pro leagues in Japan and such) but they’re very rare indeed.