It’s not as important.
Tanks need to cap spell hit not because they want to maximize damage/threat, but because they want to never, ever miss. If a tank misses with a critical ability, it can mean your entire raid wipes as the threat management strategy collapses.
Melee want to hit cap for a combination of reasons. First of all, melee have enormous amounts of critical compared to casters. While it varies somewhat, a rule of thumb is 60 Int = 1% spell critical while 20 Agility = 1% melee critical. Moreover, the melee system is a ‘one roll’ system where misses can cause you to become ‘critical capped’ - you need more hit chance before you can actually benefit from your critical.
Lastly, the scaling of attack power is 14:1. If you equip +14 attack power, you get +1 dps (before all those multipliers you get from talents, etc.).
For Hunters, you would simply gear raw Agility for the best dps. However, it’s not actually all that much better than making the almost trivial effort to hit cap and Hunters are more of a ‘utility’ class than a pure dps class in Vanilla. Given the choice between an almost imperceptibly smaller amount of dps so they can fit another +3% hit chance (on top of talents/enchants) and missing Tranquilizing Shot, most raids are going to choose the former.
Now, look at casters. They very rarely cast ‘must land’ abilities. It’s nice not to miss with Polymorph or Banish, but you can simply cast them again immediately - it’s not like Sunder where you need to wait on Rage or Tranquilizing Shot where you’ve got a 20 sec cooldown. You’re also not casting these abilities on raid bosses, but on lower level (and thus easier to hit) minions. They don’t have a ‘one roll’ system where critical is limited by their hit rate and even if they did they don’t have enough critical to worry about it.
Lastly, the scaling of spellpower is 3.5 Spellpower = 1 dps (again, before various talent multipliers).
While spellpower has a slightly (72% for pure spellpower, 40% for single school spellpower) higher itemization budget, this doesn’t come close to making up the disparity between spellpower and attack power.
This creates a situation where spellpower is the dominant caster stat by a huge margin. There is no point at which an equal iLvl spell hit/critical piece is better than its spellpower counterpart. If Warlocks could equip pure +shadow damage in every slot, that’s what they’d do (the same with Frost Mages and +frost or Fire Mages and +fire), happily ignoring hit and critical entirely.
But casters can’t actually do this. While pre-raid BiS manages to do well for a while, eventually the itemization budget for epic and higher iLvl items just swamps the value of wearing pre-raid BiS items - especially when you consider all the additional Stamina and mana provided by those raid items.
So the question really isn’t “how much spell hit do I need?” but “how little can I get away with without avoiding otherwise incredible gear?”.