Because it is not meant to raise the entire cage's humidity. It is a place the snake can go to get the humidity it needs.....which remains in the hide. Being a snake stays in the hides the majority of the time the snake gets the humidity it needs even though the rest of the cage may be low.
My humid hides were nothing more than a regular hide with wet moss in them....I still do it this way. Works great and nothing special is needed.