Because its a faulty assumption that a lot of people have that there are no consequences for botting. If SE did nothing about bots, and I mean nothing, there would be more bots than players by this point. We would see bots that were here day 1 still here. But we dont. We dont see every single chat spammed to the max with RMT spam. We dont see them every 5 seconds no matter where we go. Do we see them? Sure. Is there some RMT spam? Sure. But people seem to be thinking that this game is flooded wit bots and theyre everywhere. The oddity of seeing them is proof enough. If bots were everywhere, this wouldnt be a spectacle. This would be "Its another day on FFXIV." The fact people pause and go "Oh that must be a bot" denotes that it is not a common occurance. It's an oddity.
So yes, SE does actually take action against them. How harsh or proactive is debatable. I would imagine the best way to get bots is to have them purchase an account and pay for a sub then find and ban them after 24 hours. Punishing bot players shortly after they activate their accounts allows SE to pull money from them while dealing with the bot issue. It becomes not as profitable to be a botter as a result. That is how I would look at it, and would make sense in that light as you are more likely to see low to mid level bots, not max level bots (typically).
TLDR though: SE does take action and its a player base 'myth' that SE doesnt do anything about bots. They do. That is simply the fact of the matter. How effective they are is a different question, however.
You assume that people trolling cant set up bot scripts themselves or make mistakes. If it is an RMTer, then thats bad news bears for them, but Im still more of the mind set that its not. Particularly since bots can zone out of cities and maps via teleport or other means. Just color me as skeptical.
Consider that if this is a bot net for RMT, thats how many bots? 40? 60 maybe at most? On a server pop of what? A 8 thousand? 10 thousand? 16 Thousand? Thats is a drop in the bucket in that case, and all youre seeing is a hard concentration of them. Makes it easier for them to be banned, but again, this goes back to the what I was saying: This is not as bad as people think it is, and saying SE does nothing about bots is a myth. I will guarentee that every single one of those bots will be banned within 48 hours. Every single one. That doesnt stop a RMTer or Troll from making more. But that then gets into teh discussion of how effective SE is, not whether they take action or not.