To put it simply, because even though they're comparable and complementary even, they're not the same, and also, NWN doesn't work that way, I'm pretty sure.
On the first point, multiclassing for any class combo, even when the classes complement each other, is still a sacrifice in the progression of the the original/main class. A good example is Wizard and Spellsword. The latter adds a "synergy" bonus that boosts the effective caster level of the Wizard, but Spellsword levels taken prior to level 21 do not count as Wizard levels.
If you notice, there's a difference between an Epic character and an Epic class. The Epic character is any character past level 20. For the Epic class, the character must have 21 levels (for basic classes) in one single class to be considered Epic (and subsequently receive the Epic class feat as a marker, such as Epic Wizard).
A Wizard who makes it to Level 21 with some Spellsword levels prior to that will not be an Epic Wizard, and that character will not have access to the Epic Spells until 21 levels of Wizard are gained.
Anyway, multiclassing is a sacrifice. The path of a Bear Warrior is not quite the same as that of a Barbarian. The Bear Warrior diverges his/her training to be something else. A pure Barbarian of the same total character levels would be (generally) a more powerful Barbarian than the Barbarian/Bear Warrior split, but the latter character would be more rounded and have some advantages that the pure Barbarian does not. The same is true for any other such combination where there's synergy like that.
