Why can a partial class only contain partial classes?

Question

I struggle to understand, why a class with the partial modifier, can not contain classes that are not partial. They don't need to necessarily have their definition spread across multiple files.

Conversely, to me it makes more sense to require classes containing a partial class, to also be partial. But this is not the case, even though a class can't have definition in another class, without its parent class also being defined there.

Am I just not understanding partial correctly?