It's true that no one respects Combat mentors, because many (not all) have no idea how the game works, picked it up for the crown or mount grind and faceroll dungeons are not a measure of player knowledge.

But ...no one respects skilled players who know what they're talking about either. They just see them as sweaty tryhard elitists and take advice as a personal attack. They don't want advice, they just want to mash random buttons or spam Cure I and be praised for it.

This game is split between veteran players and casuals and both sides strongly resent each other. At the same time the casual side doesn't want to take advice from a fellow casual either or respect it (unless the fellow casual is telling them they're doing good). Since the game constantly reinforces the mentality that "ff14 is casual, play as badly as you like, it's a good thing not to care!" the only people who need advice are conditioned to resist it.

Thus there just isn't a need for mentorship.