I'm not sure they do, technically you get recruited by Lae'zel on the nautaloid. It's just once she recruits your character, you the player get to control her.
She only joins your character later due to your character helping her.
Shadowheart is a reluctant follower who is a bit desperate and lost - You're the closest thing to help she's seen all day.
Gale actually seeks you out, he's specifically looking for others with the tadpole issue - Not only does he want to join you but is upset if you don't let him.
Astarion, one could assume is looking for a reliable source of nourishment, he does also need some convincing.
There is also the whole tadpole thing, notice how every follower gets a pretty strong tadpole reaction before they agree to join you.
