Theoretically, it doesn't, but practically it does; At least it is my experience.
It very much depends on how it's implemented.
Think of games like osu! or Geometry Dash. They are "Simple" in that they only use 1-2 inputs. But they're far from boring because what you do with small number of input options are very much interesting.
While in games like WoW, FFXIV and basically every MMO, they reduce the number of abilities you use... But don't' do anything special with them so you're just playing a less interesting character.
Just like there's the flip side where complexity doesn't necessarily equate to more enjoyment.
Having ability bloat so you're swimming in lots of actions that don't do anything unique is not interesting, it's simply a waste of time.
Having systems that are so complex you need a PhD in theoretical physics just to understand what's going on does not make them more engaging.
"Simplifying" a game can be just about removing redundancies that serve little tangible purpose.
For example, what was being gained from needing to get Point Blank Shot and Precise Shot feats just to be able to use ranged attacks against a target that is next to an ally without a huge -4 attack roll modifier?
What was gained from having Cantrips being utterly useless because they were just 1d3 damage spells necessitating every caster to run around with a crossbow?
These sorts of things don't add anything but unecessary "Complexity" to the game. They're not creating any meaningful decision making.
So you answer why bg3 was a success is caused by several factors that wouldn't meant success if other game did what larian did.
Same goes with witcher 3, I have no clue why witcher 3 was a big success on western markets and witcher 1/2 didn't achieve the same success despite witcher 1 had better dialogue from my point of view.
If we look at other games that managed to be more successful than their typical niche;
Witcher 3 had a smoother combat system compared to 1/2. This made it more enjoyable to actually play (I personally couldn't stomach the jank combat of W1 and W2)
Mass Effect 2 had a more streamlined, action orientated combat system which lead to less time spent faffing around in menus and more time shooting aliens and wooing sexy companions.
Dragon Age: Origins only notable things were the shift away from the "Classic" CRPG style of game (Isometric view, low res character models, PNG portraits) into the more detailed character models from a closer viewpoint (Enabling sexy character designs)
Elden Ring became more accessible. Since its open world freedom (And powerful mechanics like OP Ash summons, OP status effects and OP spells/weapon arts) enabled people to simply go elsewhere if they found content difficult which was a change of pace from their prior titles more linear paths (Also, this lack of linearity also meant less traps. Less ambushes from around corners/behind barrels, no mimics etc. Since these things don't work with freeform paths. Without traps, there's less frustration for unfamiliar players who'd get surprised and die, losing all their Souls/Runes)
So we can look at BG3 for what things it seems to do:
- It also uses the more "Modern RPG" style of character design and camerawork, enabling sexy characters just like DAO (And unlike Owlcat's offerings which still rely the classic style. Even Pillars 2 is closer to the CRPG style than the more modern style)
- It has a smoother and more accessible combat. Between the usage of 5e and Larian homebrew, it's more straightforward to do combat than many other titles, there's no unnecessary bloat of required feats and building a character is as simple as slapping in some stats and picking a class that sounds fun (While hardcore RPG fans can sink their teeth into multiclassing and min/maxing for ultra powerful builds)
- It gained a massive boost in popularity since it piggybacked off a well regarded IP. Not only hardcore RPG fans like the Baldur's Gate name, but many casuals likely have nostalgic memories about playing BG1 or BG2 when they were young.
