Mudra why so many ninja don't use them?

[Jeeqbit]

What I do see too much of is Doton on single-target bosses.

That's because it used to be slightly better to use it on a single target (I think by a single DoT tick). It seems that SE fixed that oversight.

i was confused what my tank stance did i kept spamming it off and on like a random cooldown

I've seen tanks do that occasionally. Rare, but amongst my thousands of dungeon runs, I've seen it multiple times.