Finite data types are, or are isomorphic to, (constant) relations, they aren't relvars like tables/views (relations include both constant relations and relvars.)
> What do you gain by calling types relations
The fact that everything true of other relations is true of finite types.
Relations are commonly used when referring to tables and views.
What do you gain by calling types relations while introducing confusion and diminishing practicality of the relation concept?