I think it's just possible that wizards gain new at-will powers even less frequently than once every 6 levels. It's quite possible that the at-will powers you do have, like magic missile, scale by level, so that you only gain new ones every tier or so. So heroic tier wizards typically use magic missile or flame burst, paragon tier wizards use something else and epic tier ones use something else entirely.
I mean, when you look at the at-will attack powers, wizards don't really do anything but do damage. They have one for single targets, magic missile, and one for groups, flame burst.
Now I think that it's possible other classes gain at-will powers more frequently. After all, the wizard is a "controller" so he might get utility powers more often than the other classes, who gain at-will and attack powers more often. There's a number of ways the designers could have it set up.