This is just not true. In fact, it’s common to decrease/damage/destroy your run differential by adding hitting because the new hitters are worse defensively.
And hitting isn’t more the more consistent aspect. Players have bad seasons at the plate, too. Not to mention, hitting isn’t the weaker part of the game as hitters are at the disadvantage. Theres a reason no hitter has ever batted .500. Maybe there’s more room to improve upon hitting because of the inherent role it plays in the nature of the game, but because those same players play defense, it doesn’t necessarily impact the run differential positively…