Call site is tricky because you need safety guarantees about the underlying data for it to be a value type. For example, it needs to be immutable.

If you were to call a function with a site declared value type, the function you call would have to have syntax as well to indicate "this is a value type I'm receiving and not a regular type" so that it doesn't do an illegal operation with the value type.

I think it keeps going back to "preserving type invariants". Turning an object into a value can have surprising constraints on the integrity of the type. These include: forced empty constructor (for bulk initialiation in arrays), violation of cross-field invariance (think of a Range-type where "end" value is suddenly smaller than the "start" due to a data race), etc.

Java always had a focus on enabling library authors over empowering call-site users. And I think approach has proven its dividend.