isn't "why" the Head?
If it is, oughtn`t Adjective Clause be a Pronoun or Adverb Clause? "why" is a Relative Pronoun or Adverb?
In this Adverb Phrase as Adjunct Adverbial, "in" is the first word. Thus it looks like the head of a "Prepositional Phrase", even though the only content word is "opinion" (a noun)?
Another [Prepositional Phrase as Adjunct Adverbial] (https://parentingpatch.com/using-prepositional-phrases-adjunct-adverbials/):