There is some stupid people out there, and not all spirits are nice. If everyone knew the existance of the spirit world, hundreds of people would be trying, and many managing, to summon spirits without good in mind.

Also, the world is on a delicate balance, and I believe that belief is more powerful then you could emagine. If everyone believed in the spirit world, it would most probably upset that level of balance. The human race has unbalanced nature enough.

And the majour countries I'm talking about are the ones who have scientists, and listen to scientists to their every word.
If a scientists told the populous the moon was completely made of crystleized carbon, there'd be people trying to get to the moon for a peice.