Watch any ballgame, and you’re sure to see baseballers chewing tobacco, chewing gum, chewing and spitting sunflower seeds, etc.
While some athletes chew gum in other sports, baseball’s level of oral activity is unprecedented. Why?
Is it simply mired in tradition now? If so, how did it become the way it is?
I was told that gum was to keep the players mouth wet.
The rest, I have no idea.
Baseball players have to spend relatively long periods doing nothing compared athletes in other team sports. When they finally see some action, usually no one is trying to hit them, so swallowing gum/tobacco/seeds isn’t a problem.
Primarily, I think baseball players split because when they start playing, they see everybody else doing it and they want to fit in.
I’m wouldn’t be surprised if a lot of baseball players do coke - standing in the outfield, staring at those long white lines… 
Seriously, I think BobT is right - you spend a lot of time doing nothing, and chewing breaks up some of the monotony.