Linear storylines and sidequests, are often used as interactive tutorials. They give the player some structure within the game. For example, in Dying Light 2, a level 1 player is partitioned off from level 6 areas. A level 1 player facing off hordes of level 6 zombies, volatiles, etc. will have no shot. That would be frustrating for players not explicitly looking for that sort of experience.

In soulslike games, it's expected that your hand isn't going to be held and you might blindly walk into areas for which you're severely under equipped. That's a part of the experience, which again, is expected by players when they load up Elden Ring for the first time.

TL/DR: Elden Ring can have an organic open world without linear quest elements, because that's what players expect when picking up the game.