[dcl.init]/7.1 says:

To default-initialize an object of type T means:

• If T is a (possibly cv-qualified) class type ([class]), constructors are considered. The applicable constructors are enumerated ([over.match.ctor]), and the best one for the initializer () is chosen through overload resolution ([over.match]). The constructor thus selected is called, with an empty argument list, to initialize the object.

This usage requires that () be a valid value for the grammar term initializer. This is not the case according to the initializer definition:

initializer:

• brace-or-equal-initializer
• ( expression-list )

expression-list cannot be empty, so () is not a valid initializer.