Sorcerers get cantrips that are infinite like a wizards yes.
Also, most spells (at least used commonly in combat) take a standard action to cast and provoke attacks of opportunity
The AoO is the "oh no, its casting something, smack it hard, NOW!" moment. but only melee attacks can be AoO (some exceptions) thus as long as a caster doesn't sit in the enemies reach, they are more or less safe from AoO
(held actions for when someone tries to cast are another story)
Remember a standard action is roughly 3 seconds long, and I don't care who you are or what you can do, 3 seconds is not that long
(side note, magic missile does not have a roll to hit, or a save, only Spell Resistance blocks it, and even then, not always)