See this is why I think the TUTORIAL should be levels 1-3 and the midpoint of chapter 1 should be level 5. Levels 1 to 3 in D&D 5e are babbies first adventure, very little complexity and designed to get to know your characters.
the vast majority of all 5e games are levels 5 to 9, because that is where the "meat" of the experience is. The life or death battles, no easy resurrection, and the most well balanced combat
BG3 is going to be the introduction to 5e for a LOT of players. Larian probably does not want to overwhelm new players with the additional complexity and choices that you'd get if you were creating level 3 characters and needing to pick subclasses.
BG3 as it stands ISN'T 5e though, for example, in 5e, having a 5 foot height advantage doesn't trivialize a combat encounter like it does in BG3. Like, the greater invisibility (fourth level spell) spell is going to be pointless because you can just levitate 5 feet up into the air and get the same mechanical benefits with the second level spell levitate, assuming you cant find a rock to climb up
