Even if the OP was specifying pitchers it wouldn't make a difference as to the answer
You mention Outfielders OP which actually quite interesting since I used to be an Outfielder when i played the game community wise.
One of the repeatedly emphasized deals was not only learning how to throw the ball correctly and accurately but conditioning yourself to the nature of the baseball. They gave us tennis balls, softballs, plastic balls with Pennies inside of them and then actual baseballs and watched as the different weights under different angles and fielding situations wrecked most plays or perception. This goes double for OF.
However, the throwing is the more interesting part. Judging distance isn't too bad of a job, but like in the video above, baseballs get flung around in a very specific manner. This manner was pulling your entire arm back and forming a pseudo 90 degree angle with it behind you. hand forward elbow back. When you are throwing the ball, you have to bring your elbow forward followed by your hand and then forcefully flick the ball forward which would give it the strength, momentum and keep it as accurate as can be.
Everytime you have to make a throw like this, especially in play assuming there is a chance to get someone out from beyond the infield. You stretch your shoulder, your elbow gets tugged on and your wrist bears the final brunt of all that momentum as you throw. Regardless of whether you had to actively field the ball or you just waited for it to come to you/had a more casual pace. I screwed my shoulder a bit only once and that was just little league stuff. Players in general are doing this almost every day for at least 2 hours a day and a non trivial amount of time out of time zone, so you can factor in some potential jet lag.
This is also not taking into account that Innings aren't timed like other sports are. A single inning will go on as long as the number of outs are under 3.
But I digress.