Because the boosted strength is a magical effect - put the character in an anti-magic field and it goes away. If a Barbarian had 26 Str (possible with the appropriate Tome or Libram) then that would count as natural, because it wouldn't go away in an AMF.
Yes, I know it is a magical effect.
My question is - how does the sword know that?
With the magic weapon, there's some indescribable field of magic about it that makes it hit better. It isn't just that it is sharper or something distinct and physical (or else you couldn't have, say, a magic mace or club with no edge). And the Guidance spell creates a similar effect. And we can imagine that these fields don't add together. That makes sense, in the game world.
But, that belt doesn't make some indescribable magic field. It actually makes the character stronger, something distinct and physical. It is hard to see why that indescribable field has a problem with the distinct and physical effect. I mean, if you had a spell that made a character taller, would you say their sword couldn't reach up into a tree to hit someone, because the magic of being taller couldn't interact with the +3? Why is "being magically taller" okay, but "having magically mightier thews" is not?
Moreover, consider: You have a +3 on your sword. You have a +2 on your shield. The sword affects the sword. The shield affects the shield. And you are fine. But the sword affects the sword, and the belt affects your muscles! Why doesn't this work?
Your answer is effectively that it doesn't work because the sword and belt both impact the same *die roll*. But the magic happens in the game world, the die roll does not! Magic doesn't actually know about die rolls! The impact on the die roll is merely an abstraction of the impact of magic in the game world, and is thus a poor justification for how magic works in the game world.
Other editions have handled this by saying that magical bonuses *of the same type* didn't stack, and went to the length of making up this huge list of bonus types. This put things in a logical framework that made sense, but was a pain in the neck and a source of mi-maxing. We probably don't have to do that far. We only have to go so far as to answer the question, "Does it make sense *in the game world*?"
So, answer me - *in the game world*, why can't the magic strength and magic sword stack?