Simply stating "there's too much mobility" isn't actually explaining, it's stating an opinion.
Moreover Karthus has a wall that slows to make targets less mobile. They are meant to be used in tandem. You can also force enemies into a direction via aura or aa, and Q where they are likely to go.
I assume this is happening during the laning phase, as that's the time people are going to most often dodge Q. In which case instead of using it to try to hit the enemy, it's more effective to use it to deny them cs. Also, most champs don't have "absurd movement speed" during the laning phase.