One reason outfielders don't have stronger arms might be they don't practice as much as we did. Most teams today don't take outfield practice. Another reason is baseball has to compete with other sports now - basketball football soccer - for the better athletes that might have more skills and stronger arms.

The phrase 'off with the crack of the bat' while romantic is really meaningless since the outfielder should be in motion long before he hears the sound of the ball meeting the bat.