You have a point. Most of the followers we gain through these sites unfollow you once they earn credits. Secondly, these accounts are fake and created just for the sake of following accounts so that they can earn credits. Since they are fake accounts, they do not interact with our content and we do not benefit even when we have a large following.It's true that will you build a huge number of followers, but i don't recommend those exchange websites for 2 reasons :
1-Lot of people cheat in there, after following them, they remove their follow.
2-Fake followers, which means even if they keep following you, they won't interact with your content because they might not be interested