Why can't anyone throw a baseball faster than 100 mph?

Why do sprinters keep getting faster while baseball pitchers seem to have maxed out? Because track athletes don't approach the limits of what human tendons and ligaments can handle. When you run the 100-meter dash, no single stride represents as violent a motion as the arm makes during a single overhand pitch. Sprinters can build up their muscles without worrying that the extra force will rip their ligaments apart—that's why steroid use seems to make sprinters faster but won't help pitchers generate velocity beyond a certain point. (A better reason for a pitcher to take steroids would be to decrease the time it takes to recover between games.)

Link