Pitchers wear high numbers for one simple reason: there’s so darn many of them.
When Spring Training comes around, a major league club brings in the existing team members and also invites a bunch of minor leaguers to try out for the team. When they hand out spring training jerseys, the guys already on the team wear their normal numbers. Then they assign jersey numbers to the various invitees, among which are a whole lot of pitchers.
They don’t hand out certain numbers to the guys who are trying out. Some examples:
• numbers already assigned to players on the 40-man roster
• certain other unassigned numbers that they hold back for one reason or another, like #24 in Seattle. That was Ken Griffey Jr’s number. He’s no longer in Seattle, and the number isn’t retired, but it just wouldn’t be right to give his number to somebody else. Likewise Jay Buhner’s #19, or Edgar Martinez’ #11. I think the Mariners also hold back Alvin Davis’ number, but I can’t recall what it was. Ichiro wears #51, Randy Johnson’s old number, but he and the team first asked Johnson if he minded. If Ichiro hadn’t already been a superstar in Japan before coming to Seattle, he probably would have been asked to pick a different number.
I would imagine that preference is given to the player with more seniority if two players want the same number. A player with previous major league experience will probably want to wear the same number he wore with his previous team. If it isn’t available, he will often pick another number that is somehow related to his previous number. For example, Alex Rodriguez wore #3 in Seattle, but obviously couldn’t wear that number in New York, so he picked #13. Heck, Randy Johnson couldn’t have #51 when he went to the Yankees, despite his status, because Bernie Williams had it. So he picked #41.
So when you’ve got a bunch of minor league pitchers in spring training, there are a lot of people ahead of them for first pick of jersey numbers. They may want to wear whatever number they wore in high school or college, which was probably a low number, but it isn’t available and so they take what they’re offered: a high number. If they happen to make the big league club, they’ll likely wear whatever number they wore in spring training. And then carry that number throughout their career.
I think there’s also a bit of ranking involved for the rookies: the lower your number, the better chance you have of making the team. By that, I mean that the team assigns lower numbers to the players they think have the best chance of making the cut. Every once in a while, an unlikely prospect has an outstanding spring training and makes the big club. Baseball superstition makes him think “57 is a lucky number!” and so he wears the same number on his big league uniform.