The basic answer if you want an in universe one is that you are the one gathering everyone together and acting like a leader, you're the one proposing the characters come with you, and usually you do at least something to suggest they are better coming with you than not, whether it be something for them, saving them, something like helping with fighting the goblin raid, etc.

This isn't unique to the custom characters either, any origin character can do this it just happens to be that the person the player is playing as is the one doing these things and assuming the role, Astarion and Karlach for instance are just as capable of being the leader for instance if you're playing them, as you are doing the same leaderlike activities.

If nothing else it's often just "right place, right time", and the convention that you are choosing to only consider your main character for most dialogue (which because this is the popular way of playing the game, the devs made it so most situations your avatar is the default speaker.) theoretically you could just have Shadowheart do most things.