You know, it's really amazing when I think about it, but the modern ballfield is the most perfect setup one could devise.
Ninety feet between the bases, and the throws usually arrive about the same time the runner does. If it was farther, say, 100 feet, the batter would be out much more often than not. If it was eighty, he'd get to first base almost a the time. But ninety? Perfect.
Similarly, the distance from the pitcher to the hitter. That pitcher is throwing a ball that sometimes reaches 100 mph! That doesn't give the batter much time to see where the ball is going and to try to take a swing at it (This also explains why curve balls and sliders and the ike are so effective - the batter has to commit to swing long before the ball actually arrives, and if the ball's not where he thought it was gonna be he's outta luck.).
The pitcher is just far enough to make things interesting. Farther out, and he'd lose speed and more importantly, the batter would gain time. Any closer and no batter other than the Flash would have enough time to decide whether to swing.
The nine players on defense are spread out around the diamond just enough so that it's possible to catch nearly every hit, and it's also possible to hit into areas where the fielder can't make the play. Again, any change in the number of fielders and you wouldn't have this balance.
On any day, any team stands a chance to beat another team - or be beaten.
It's truly the perfect summer game.