I see it like this, the Lord's and their souls try to accomplish something big and fail thus losing themselves in the process. The best example would be the king in the graveyard structure (I forget the name of the place) he is just casually walking around the room and sulking. While his personal guard is still standing guard of him. It seems the bigger the task the more you lose yourself so for people with less grandiose tasks, they tend to retain their soul and it's strength for longer.

Souls final bosses have always somewhat been easy, I do know since then Bloodborne has had a "mostly difficult" final boss but idk why souls games don't.