I think "because mmo" is a copout answer. You're working with a lot of other people in a zerg situation whether you're in a party or not. It's not a meaningful party relationship; typically people will spam asking for an invite then drop party right after hunt is dead.

Seems more fair weighing individual contribution instead of whatever the random people you were paired with did. But if we aren't encouraged to form parties then it's less likely for non-healer players to receive healing or res, which is the best reason I can think of to remain the way it is.

Regarding solo players seeing different results: the total number of players participating and the amount of time the enemy is alive are factors in how much you're able to achieve before the end.