it's not just cheaper, its eminently more reactive.

your human recruiter is going to have their own biases, could be tired, may not know how to deal with personality X or Y, they may not learn.

the bots can track how they do by upvote, react quickly, won't get tired or annoyed, and can use existing data to filter and target those who are extremely vulnerable.

hell, with real-time analysis by data mining companies (cough cough FB, reddit) they could even figure out if/when a normally stable-ish person is in crisis or has a bad week, and hit them at their most susceptible. 99% of the year they might be able to see through the BS, but this week things are hard and what they're seeing in their feed just feels right, for once...