It's a product of the two series telling different stories. Zelda is pretty plot focused with memorable scenes and iconic events. Dark souls is all about the characters, not only the NPCs who inhabit Lordran, but the bosses, too. Dark souls feels like a completely different game when you understand who you're fighting and why they have conflicting goals to you. Meanwhile zelda characters are just there to play their roles. Take koloktos from skyward sword and the gaping dragon from DS1. Koloktos we know nothing about, other than it being an obstacle for link. The gaping dragon however is known to be one of the few dragons remaining in Lordran. We know that death is both aware of and interested in the gaping dragon due to the presence of the channeler above the boss arena. We know that someone, possibly an important person, died fighting the gaping dragon thanks to the clothing found in the room. This kind of obtuse storytelling is omnipresent and unique in dark souls, and seeing it in other games would honestly make dark souls less special