Slow projectile speed? It has a 2000 unit projectile speed, which is equal to Ezreal's Q, which I've never had any trouble landing consistently.
And if the hitbox was too big, it'd be more likely to accidentally clip a minion that you weren't meaning to target. I know that I use to either farm the front & back minion line, or to hit the front line and harass the Champion standing behind them. So I like being able to snipe it between a few creeps and hit my target.
Seriously, most Veigar mains I've spoken to have come around to the Q after some practice, and I (though I'm not a V main) enjoy it.