Board Thread:Questions and Answers/@comment-35027309-20180319020516/@comment-24577221-20180319042829

Because the game's makers want to give advanced players something to work towards, to keep them playing.

Several of the decorations are extremely expensive, and only become available when a player reaches a high level. But they are the only way of getting some monsters to 100% happiness. Again, that gives advanced players something to work towards.