I don't really know where you guys have seen that it doesn't stack.
As far as I'm aware magic resistance stacks to 85% , and individual resists stack to 85%, all additively.
Example: You get shot with a fireball ( lets say 100 fire damage for the sake of the example).
You have 85% magic resist , meaning that you would get ((100-85)/100) * Damage (100) = 15 fire damageÂ
This is further reduced by individual resists, meaning that if you have 85% fire resist , you get another reduction : ((100-85)/100) * Damage (15) = 3 Fire damage , this would be the final damage.
And overall 85% magic resist + 85% fire resist = 97% overall fire resist.
Hope it's been helpful.
This is all according to my knowledge (and common sense) , if you've got better info please let me know