Because, to put it simply, the ultimate endgame of Book 4 was something in which strong spiritual powers and knowledge contributed precisely zilch to the good guys' objective.
In the case of Book 4, the objective was both purely military and rooted entirely in issues of the material realm, rather than the spirit world: Stop the giant mecha-suit, disable the cannon, defeat Kuvira. The fact that the cannon used spirit vines as an energy source is purely incidental to that objective - unlike with, for example, Unalaq/Vaatu in Book 2, the threat was 100% physical - no spiritual aspect to it whatsoever.
And the spirits had already decided not to get involved. Could Jinora have helped there? Maybe, but not very likely IMO.