Because they get tons of “messages” from women who are supposedly interested in them.
They also often ask questions, and then (without discussing your answer) immediately go into telling you about their view on the matter, so you feel as if there is some meaningful conversation.
This removes the need to pay substitutes to type messages, any human error factors. There are plenty of ads on social media sites (VK, Facebook) offering chat bot programs for purchase, written for specific PPL sites.
Modern bots are quite sophisticated: They have time delay functions and may sound intelligible in their messaging.
PPL sites constantly make as if they are trying to “catch” scammers (typing slaves-substitutes).
Although stopping this whole problem is very simple: Stop charging per message and don’t pay commissions to agents.