Three relatively simple reasons:
1) Square Enix doesn't accept any evidence they are not fully in control of as a basis for disciplinary actions. No screenshots, videos, streams or anything. All of those can potentially be faked and they just don't want to deal with the ensuing mess that acting on such evidence can cause. In practice it leads to in-game chat logs being the only reliable source of evidence they will consider.
2) Square Enix has no interest in developing or utilizing an anti-cheat software that checks what are you running on your machine. Both from moral and legal privacy-protection standpoint. Paired with the previous point it leads to all the rules you mention being entirely unenforceable outside of players blatantly bragging about their third party tools in in-game chat.
3) In the end, to please the most players Square Enix are actually pretty happy maintaining the status quo. Going hard against any third party software alienates the "hardcores" who are looking to review their performance. Allowing players to openly talk about dps numbers and exclude players based on them upsets the "casuals". The current situation with third party tools being a "grey zone" and following "fight-club" rules where mentioning them in-game will lead to trouble but actually utilizing them for personal improvement is allowed to stay under the radar is agreeable if not perfect for most players.
