For me, that is *exactly* how it should be for casting a fireball, where making that judgment, in real time, of where the enemy and your allies are going to be when the spell goes off is part of the expectations of the combat system.
But the scenario of one person managing the fireball spell and being able to devote their entire attention to managing the fireball spell doesn't exist.
You aren't solely and completely focused on casting that fireball. You're a human being with one brain and one set of reflexes trying to control four characters at the same time while dealing with 5-8 characters controlled by the computer with inhuman reflexes and reaction time. You're targeting the fireball, and you're trying to counterspell the caster, and you're trying to backstab the enemy cleric and you're trying to heal the fighter who is trying to get in an enemy fighter's face and you're trying to move everyone at the same time to find cover from the enemy attacks.