They do get banned and yes, they do create new bots too, even more of them than get banned. The thing is, along with stolen accounts (fishing websites hello), they also apparently buy copies of the game, as otherwise you can't use any trading tools at all, making unpaid bot account simply useless, and you can't steal that many accounts solely for botting (given that usually they are only interested in money character has, not in the account itself, as I can see people successfully logging back after they were robbed).
So it's kind of... weird, because as much as they are annoying and in some cases straightforward harmful, they also bring some cash to the company, and it's not like SE can prohibit people buying the game. Unless there will be some type of in-game tool to exchange real money for Gil, there's not much what they can do sadly (and I think they said that selling Gil for money officially is unlikely to ever happen for some legal reasons they have in Japan or something).