As much as I transferred my mother to Elizabeth Shore of The Black Dahlia as much as her dad mutated into an obsession with crime in general well I have thought about other things throughout the years.
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.