I'm not sure if I'm providing a good answer, but it's because there are different types of defenses. Split damage weapons need to be balanced so that they're no overwhelmingly powerful, so when a weapon has split damage, each damage type is around 65% of what a normal non-split damage weapon would be. Say a longsword does 120 damage - a lightning longsword would do something around 78 physical and 78 lightning damage. If you add the total of that, it technically does 156 damage, compared to 120 - however, that 78 physical damage is lowered by the enemy's physical defense, and that 78 lightning damage is lowered by the enemy's lightning damage, so the actual result might be dealing around 100 damage per hit. The normal longsword still has to get through some physical defense, so it would probably be around 100 damage as well.
The advantage comes when an enemy is weak to lightning, so that lightning longsword would probably get the full lightning damage, or really close to it. The drakeblood greatsword applies here; if an enemy is weak to lightning and magic, like say a Black Knight, then it would probably do really darn good damage. Of course, if one is really high level and infuses the weapon with chaos or dark, then it can be extremely powerful, as the combined scaling from strength, dexterity, intelligence, and faith would be pretty darn good.