As Rhobar said, starting characters at level 5 would be overwhelming to a lot of players. Especially since - if you're playing singleplayer - you'll be controlling 4 characters, which combined is a lot of abilities to start the game with. Starting at level 1 allows you to grow into the characters' abilities.
I'm assuming hoping praying that Larian will improve the game's onboarding (nested tooltips were step in the right direction). But in it's current state, booting up BG3 as your first experience to D&D is difficult enough while starting from level 1.
