Why is there no Nekker/Mourntart (Grave Hag) Synergy?

[4RM3D]

That said Unseen Elder's 3 consumes work with AB, so i feel the problem is more mourntart than AB.

The only problem with Mourntart, besides not being viable, is her description, which leads to unclear interactions, like these. As altaybek stated, it's counted as one consume action, whereas Unseen Elder is clearly three actions because you have to select three different targets. Somewhat related is this post about multiple actions.

PS. While Mourntart is not viable now, making each card count as a consume, would become insanely OP with AB and Nekkers. That's not the way to go about things.