Mudra why so many ninja don't use them?

[Absurdity]

Sub-optimal gameplay. In my mmorpg?!

Sub-optimal would be just using them on cooldown or using them on the wrong abilities. (like single target Doton, which often stems from the wrong assumption that DoT effects tick every second, most likely learned in other mmos.)

Not using them at all is just ignoring a main feature of your job and expecting other people to carry you because you can't be bothered.