Are factkin/factives real or not? If not, why is it such an established concept in the kin community and why would someone think they’re factkin? If they are real, then why does the community hate them so much?

Factkin are potentially real. That is, there is no reason why they couldn’t really exist. 

However, generally speaking claiming to be another living person is never anything but harmful, and potentially manipulative/abusive.

Factkin of living people are the only kin I would ever suggest that should not refer to their kintypes specifically, to avoid being found by the people that they believe they are, and causing mental problems or abuse.