Spritual Limiter

It should be noted that all Shinigami captains and lieutenants have their spiritual energy reduced by about 80% when they enter the human world by use of a spiritual limit, represented by a seal resembling the symbol unique to their division, in order not to cause unnecessary damage while there. In extreme cases where their full power is needed, they can request a limit release.

Also in the real world Shinigami can't be seen unless they are in a Gigai. They can only be seen by other spritually aware beings.