Souls is your hitbox vs. theirs in open, 3D space with free flow movement. You can dodge the attacks any way you want, you can put your weapon anywhere you want, it's just about getting your hitbox to line up with theirs. Enemies are aggressive and will independently attack and overwhelm you. Actions actually have commitment and you can't just cancel out of them with an instant parry or i-frame dodge/jump.
Most action games are fundamentally different as they are about magnetized attacks, both for you and your enemies, movement and positioning matter very little, enemies will circle around you patiently waiting their turn to attack, and you can instantly cancel out of almost any action with a contextual parry some instant i-frame dodge/jump.
Add onto that a massive variety of well-designed enemies and bosses that are aggressive and pose unique dangers with large movesets, incredible level design packed to the brim with secrets, hidden paths, and shortcuts, a generous amount of build variety and playstyles, RPG mechanics that are actually balanced and satisfying and don't lead to spongy enemies or one-shot kills, and a co-op/PvP system, and it's a no-brainer why Souls is as big as it is.