Because most fights aren't against solos, so there's usually 5 or so enemies. Hence hitting most them can be pretty good.
I'm aware of that, especially considering I made the same argument in my first post. I didn't explain that last bit about Wizard very well there, though. I threw it in with an edit, and it probably wasn't clear what I meant. I apologize... let me explain.
In PHB1, the definition given for Controller is:
Controllers deal with large numbers of enemies at the same time. They favor offense over defense, using powers that deal damage to multiple foes at once, as well as subtler powers that weaken, confuse or delay foes.
About 2/3 of that definition focuses on how Controllers attack multiple foes; this makes them ideally suited to fight Minions, and I don't think it's too much of a stretch to say that the Wizard (especially with powers like Scorching Burst) was originally designed to foil Minions, since they're the type of monster most 'vulnerable' to multi-attack powers.
The only problem is this doesn't work. Those 'powers that weaken, confuse or delay' ended up being far, far more useful, simply because Minions have a negligible effect on combat after level 3 or so. Another problem is that more and more characters are able to comfortably attack multiple foes (some even get At-Wills to do this), so it simply doesn't hold up as a character's shtick.
That's why the Invoker and Druid all have At-Wills that do things -other- than attack multiple foes. You'll see no Magic Missiles or Scorching Bursts here; the designers learned from their mistakes. And though I haven't read it yet, I'm willing to bet the new Wizard At-Wills in AP do more than attack multiple foes, as well.
Anyway, that's my 'evidence' as to why Wizards were initially thought of as Minion-poppers, and why that made their At-Wills so very, very crappy. They were designed to take out an already non-threatening foe... much like how Warlocks are designed to take out toothless, stun-locked Dragons. It'd be like creating a class with a class feature that says, "You do +5 damage on attacks against creatures that are five levels lower than you." Do you really need a character that specializes in killing things that already posed very little threat?
Warlocks need a gimmick beyond "singe-target controller" just as bad as Wizards needed a gimmick beyond "guy who can attack more than 1 opponent a round".
Being a proper Controller is about spreading nasty debuffs and statuses over multiple foes, which the Warlock cannot effectively do. A Striker is about dealing reliable damage or damage with high spikes, which the Warlock is not able to do as effectively as other Striker classes. I'm not saying Warlocks are useless... I enjoy playing them myself. But I think it's clear they have some issues.