If saints are in heaven, why don't we say St. Moses or St. Isaiah?

It is simply the manner in which the traditional use of the word has evolved. In Western Christian tradition, Old Testament persons are not referred to as saints (other than the names of angels mentioned in the Old Testament). The word was applied to early Christians who were considered extremely holy and never evolved into being used for pre-Christian persons.
Clearly, certain Old Testament figures are considered to be among the saints of heaven. Moses and Elijah, after all, were seen at the transfiguration of Jesus. As the Catechism of the Catholic Church notes:

The patriarchs, prophets and certain other Old Testament figures have been and always will be honored as saints in all the Church’s liturgical traditions (61).

Eastern traditions have used word titles reserved for saints and applied them to Old Testament figures, so it is not a universal practice of the Church to avoid calling them saints. Also, the Roman Martyrology does include some Old Testament figures.
It is merely an accident of word evolution and application in certain parts of Christianity that Moses and Isaiah are not called St. Moses and St. Isaiah.