Here's why:
* Prepositional phrases consist of a preposition followed by its object (a noun, pronoun, or phrase). The preposition establishes a relationship between the object and another part of the sentence. Examples: *in the box*, *on the table*, *with a smile*.
* Linking verbs connect a subject to a subject complement, which renames or describes the subject. Examples: *is*, *seems*, *becomes*, *feels*.
Linking verbs cannot be prepositions, and therefore cannot be part of a prepositional phrase. They are part of the main verb phrase, which is the core action or state of being in the sentence.
Example:
* Incorrect: *The book is on the shelf. * (Here, "is" is a linking verb, not part of the prepositional phrase "on the shelf")
* Correct: *The book is on the shelf. * (Here, "is" is the linking verb, connecting the subject "book" to the subject complement "on the shelf".)