Mortars miss; suicide drones don't. And even the crappiest combat FPVs outrange mortars at 10+ km. The second drone gets through the net. And I wouldn't be surprised if US mortar rounds cost more than Ukrainian drones. But certainly they are synergistic, and tanks and IFVs are also widely fielded on both sides of the war.

The obvious thing to do is to guide your mortar rounds with canards mounted slightly forward of the center of gravity, but the distance between "obvious" and done has a lot of rotting corpses in it.