There's a combination of reason behind this.

1- The game's difficulty comes from doing your rotation as you handle the mechs of the thing you're fighting. The GAME, however, does a piss poor job at teaching people the assumed "proper rotation" that the devs use to balance things from expac to expac. They make next to no attempt to spell out the proper rotation to people and iirc it's in part duo to "allow players the freedom to come up with their own, possibly better rotations" or w/e but most of the time you just end up with Blms doing next to nothing or Mnks doing single target rotations on a pack of 10 mobs. HELL, healers legit deal more damage than tanks ... yet we all know how common it is to get a healer who does zero DPS and just stands in a corner and heals because "That's my JOB!".

2- People feel like the time invested in the game gives them proper knowledge of it. When the game it's self doesn't teach you how to properly play your job no amount of in-game play will likely give you the full picture alone unless you're dead set on crunching the numbers ... so if you want to be better or git gud you have to look online for help. But .... most things in this game are face roll easy. So unless you want to do Savage, ultimate's and Exs you don't even need to be "ok" at your rotation.

So until the devs address this you just have to, like, focus on yourself and ask people if they're open to advice. I started this game as Tank and people's suggestions allll the way back in ARR helped me a ton before I started watching youtubes and such for information.