Answer by adjan for Null coalescing operator (??) with return

Well, because no one has implemented it this way. Or, more precisely, because return is not an expression.

throw used to be a statement only prior to C# 7.0, but then was extended (due to a proposal) to be an expression as well (only expressions are supported in the null coalescing operator).

So unless no one suggests to make return an expression, it won't work.