These threads keep popping up because FF14 is a very popular MMO with a very large player base. This coupled with the fact that people in general are far more likely to complain about something versus giving a compliment. This is just human nature. So yeah it's more likely we see threads from a few individuals complaining about mentors, but we don't hear about people peoples positive experience with mentors being posted as often even though people do have positive experiences more often, whether they realize it or not. Probably easier to recognize the bad over the good. That and there's no curated barrier from becoming a mentor. Someone who just plays the game regularly, especially if they primarily player as a healer or a tank who also leveled one of each role for something other than mentor purposes such as having the ability to fill any role in a raid or so they could get role quests done, or maybe so they could the all jobs 80 mount, or maybe they just like seeing all their jobs at 80. Anyone can become a mentor given enough time, you don't have to work for it specifically and most mentors didn't work for mentor specifically, they just played the game regularly and happened to meet the requirements. The small percentage who worked specially for mentor is not the normal, but even from this small percentage of people, I would think most came out to be ok mentors, as in not bad/rude mentors. There are more people who meet the requirements for mentor than you probably realize, and many of them choose to not wear the crown.
Also, something extra I keep having to comment on, why is it when we get these types of threads and OP runs into a couple bad mentors, that somehow those one or two experiences represents all mentors and mentors are treated as a monolith (even though we can be sure they ran into mentors they had no issue with and took no notice of, because nothing negative happened) but anyone not a mentor gets a free pass for their attitude relatively? If I run into a toxic samurai, can I treat everyone who mains Samurai as a toxic monolith? Same thing, no one is curating who can and cannot become a samurai. What if I ran into a toxic raider? Are all raiders toxic now? What about high ranking feast pvpers? Again, no one is curating these groups, it's only their own work. You may think it sounds silly and stupid but it's the exact same principle being applied.