It's all about the quest to farm sig money. Some of the amounts of money available are veeeerrrrryyyy appealing to people from countries like China (which you've already identified as a prolific country for account farmers). Luckily you can usually filter these kinds of people out through their quality of written English being extremely poor and their posts generally being off-topic and short. Patrol should get some of their posts but really, and I said this years ago too, account farming and spamming will stop when sig campaign managers become more selective. I see some are now taking a harder approach to poor English and spammy posts but not all do.