Have you never heard of the "Learning by Teaching" method?
We don't know if it will work with Hakii.
The monster trio could of been teaching the rest of the crew Haki while simultaneously learning themselves how to better apply it. Heck they could learn some interesting discoveries of Haki from the other Straw Hats that they themselves may not be privy to.
Haki blooms in battle , Let that be CoO (Coby) , CoC (Ace , Luffy).
Ussop unlocked it during a battle.
Another thing being , They are already at the peak of ot.
Luffy knows all the haki form + 3 Advanced forms
Zoro knows all the haki form + 2 Advanced forms
Sanji have two basic forms.
Zoro and luffy has nothing more to learn.
One has to be extraordinarily good at using haki to be a master.
Only examples of them are
- Rayleigh
- Mihawk
- Garp
- Dragon
Why do you think , other characters don't go teaching haki , Kata didn't teach anyone haki , and choose to protect them himself , instead of letting them learn and protect themselves...
On surface haki feels like an easy concept but
We know the level of Fishmen Island Zoro and luffy.
That too took them like 2 years to learn it and to this date they are trying to master it.
And it's not a few people suddenly learned how to use Haki out of the blue. Somebody had to be the pioneer to teach themselves to learn Haki. So where did the idea that only a master can teach Haki come from when most, if not all things, don't necessarily need such a requisite to be achieved.
You may awaken it but it will never get stronger unless you train with it more.
We know kidd have CoC , bro is a clown with no haki feat.
Law heavily relies on DF. Again barely any haki feat.
Just awakening and letting it be won't help you.
Ussop awakened it but nobody knows when did he use it again.
Zoro and luffy can't teach Ussop because that thing comes naturally to them.