Why are ults targeted at isolated units rather than casted optimally?

I think the unit prioritization of abilities in this game needs some rework. AOE ablities specifically should target large groups of units - isn't that their entire purpose for existing?

Came down to a 1v1 and my Leona ults right at the start at 1 unit on the far side of the map instead of at the units attacking her/where most of the opposing team was gathered. On top of the that, the unit was in the process of walking so she whiffed her ult completely. Cost me the game.