They had multiplayer, but games weren’t designed for it: story was about singular protagonist, and game systems supported mainly singleplayer experience.
It was completely story driven. The fact that one could tackle multiple linear adventures in any order he wished, didn’t mean they weren’t story driven. I am mostly referring to BG2, which I think, is in people minds the Baldur’s Gate2. Shadowrun: Dragonfall used BG2 structure more resently: and overarching plot for which we need to make money for, and multiple world-and-character building adventures to do that. For a more AAA example: Witcher3 - openworld adventure, which in the end, is just bunch of linear stories cleverly woven together. They just have more intertwined stories, allowing for the feel of freedom.
All games mentioned above use systems very carefully for storytelling - combat encounters, puzzles, dialogues are there placed and for narrative sake, players are pushed to engage with them in certain order. When one has a choice it is a predesigned choice, usually with a narrative impact. D:OS1&2 had little to no story, they would give players objectives, and they would give you tools and you would play with tools until you solved problem. I consider Fallout1&2 or Arcanum to be more system driven as well: if there is a door you can pickpocket key for it, pick the lock, or destroy it. Narrative games will often artificially block the progress for story reason.
I will agree, that BG1&2 does allow to mess with the system a bit more then games that follow after it. But companions with personal stories and personalities, well paced and engaging story, heroic adventure with themes well tied to the gameplay loop - this is the legacy of BG1&2, not tacked on multiplayer, or being able to pickpocket random items from peoples pockets.
