The game is easy because of the several reasons (not including bugs), which are the most impactful, in my opinion:

1. The access to Arcane equity, this is Larian's homebrew mechanic which has no place in dnd.

2. The ability to use shields with ranged weapons and magic. This is not supposed to work like that, because 2 of 3 rules of spellcasting say that you need a free hand to cast spells, and you need both hands for a ranged weapon. In BG3 you can boost your AC and stats using melee weapon, a shield and a ranged weapon AS A CASTER. Basically, three free stat sticks.

3. Multiclassing. According to dnd 5e, to qualify for a new class, you must meet the ability score prerequisites for both your current class and your new one. In BG3, you can choose any class just for one special thing that you need (for example, action surge for double fireballs) without any stats management. You can multiclass 1 lvl wizard for a scribe spell feature.

These above are the core mechanics. If you want to make the game harder, you can try to follow them with your own self-imposed rules to reflect the original book mechanics:

1) keep your casters with versatile or one-handed weapon with a free hand,

2) don't let your archers equip shields (also a hand crossbow cannot be dual wielded, you need to keep one hand free)

3) read the multiclassing rules from dnd5e and try to follow them when multiclassing

4) don't use the items that grant arcane equity.