Because typically, pre-made characters aren't designed to be min/max characters.
Not only is it somewhat unrealistic (Typical people aren't hyper specialized in their attributes), but it also keeps them from being way stronger than the average new player who has no experience in build optimization.
Also, honestly, most of them aren't that bad. Most of them have reasonable primary stats and a splash of CON to stay alive. The only exception is Shadowheart who truly has abysmal stats with her weird STR and DEX values...
I've encountered much worse pre-made character stats before (For example, Path of the Righteous has some obscenely bad companion stat distributions) and unlike other games, BG3 lets you fully respec companions so you can change their stats if you wish.
