Because of the way the classification of the relationship. In a nutshell: mousse is the mousse category under jellied treats in the frozen treats category of Western confectionery.

Cheesecake is categorized by material. Expressed as a certain amount of cheese in the center of the snack. Cheesecake can be sponge, heavy cream, chiffon, or mousse type.

The relationship between cheesecake and mousse is that mousse contains cheesecake. But there is more than mousse in cheesecake.

As for Tiramisu has been specific to the name of the snack, to the more popular process, Tiramisu has both the typical characteristics of the mousse class, but also the use of this material in the mascarpone cheese. So tiramisu belongs to the mousse class in the cheesecake.