Why can't I encode an NSValue in an NSKeyedArchiver?

I was trying to encode an MKMapView center and span into an NSKeyedArchiver for state preservation. I found a couple of handy new MapKit NSValue additions, valueWithMKCoordinate: and valueWithMKCoordinate:. Trying to encode these into the keyed archiver failed:

- (void)encodeRestorableStateWithCoder:(NSCoder *)coder
{
    NSValue *mapCenterValue = [NSValue valueWithMKCoordinate:mapView.centerCoordinate];
    NSValue *mapSpanValue = [NSValue valueWithMKCoordinateSpan:mapView.region.span];
    [coder encodeObject:mapCenterValue forKey:kMapCenter];
    [coder encodeObject:mapSpanValue forKey:kMapSpan];
}

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSKeyedArchiver encodeValueOfObjCType:at:]: this archiver cannot encode structs'

I understand that the solution to this problem is to just encode the individual doubles into four separate keys.

My question is why does this happen. An NSValue is an object, so why is it telling me "this archiver cannot encode structs"

According to the documentation of the NSKeyedArchiver class,

A keyed archive differs from a non-keyed archive in that all the objects and values encoded into the archive are given names, or keys.

In order to archive elements of a structand give keys to them NSKeyedArchiver would need metadata to know where each field of a struct is located, and what are the names of these fields. The @encode stored with NSValue gives it enough information about the layout of a struct, but the information about the names of each field is missing.

Since there is no metadata about the names of the fields in a struct, it would be impossible to archive the data in such a way as to ensure proper un-archiving. That is why NSKeyedArchiver must refuse to archive NSValues with embedded C structs.