When "Just because," is not a satisfactory answer.

Why can't a prepostional phrase be a subject complement? A question I have been trying to answer for months.
This is from A Student's Introduction to English Grammar, which is an abridged version of The Cambridge Grammar of the English Language. I am paraphrasing as the language is quite technical.

Prepositional phrases cannot be subject complements (a predicate adjective or predicate nominative). It does not matter if they are adjectival or adverbial. A complement must be licensed by the verb, meaning the complement can only occur with a certain type of verb that allows it. For a subject complement, this type of verb is a linking verb.

Here are some examples:

She is pretty. You cannot substitute a transitive or intransitive verb here: She ran pretty (Vi), She enjoyed (Vt) pretty. Neither work here. Pretty is a complement in this sentence, because you must have a linking verb in order for the sentence to be grammatical.

The book is about birds. You can substitute an intransitive: The book teaches about birds.

Jesus was about His Father's business. Jesus was learning about His Father's business.

She is from Nebraska. She comes from Nebraska. She walks from Nebraska.

She is in the woods. She sits in the woods. She walks in the woods.

She is in a hurry. She came in a hurry. She left in a hurry.

A word or phrase is only a subject complement if it requires a linking verb. If you can substitute any other type of verb, it loses its status as a complement.