It's because those abilities aren't rechecked every turn. In general, stuff that have a "use" can use calculate, because you can just calculate that at the time of use. So I'm pretty sure you can use calculate on active abilities, then again, those are just spells anyways.
Passive abilities only works with static values, because otherwise, you'd have to mimic this by "reusing" them over and over to recalculate values. You'd end up doing tons of calculations for every ability every char every turn. It's not really an ideal solution.