I was reading an article about the use of "why" as an adverb. I thought about what other function the word can have and came to the reasoning that it can be a conjunction joining clauses. I looked up a number of dictionaries:
Merriam-Webster Dictionary(yes, conjunction)
2 : for which : on account of which
know the reason why you did it
American-Heritage Dictionary (yes, conjunction)
conj. 1. The reason, cause, or purpose for which:
I know why you left.
Random House Kernerman Webster's Dictionary (yes, conjunction)
2. for what cause or reason: I don't know why he left.
3. for which; on account of which (usu. after reason to introduce a relative clause): the reason why she refused to go.
4. the reason for which: That is why he returned.
Cambridge Dictionary (yes, conjunction)
conjunction, adverb [not gradable]
for what reason:
She’ll ask why you don’t have your homework.
1(with reference to a reason) on account of which; for which.
‘the reason why flu jabs need repeating every year is that the virus changes’
1.1 The reason for which.
‘each has faced similar hardships, and perhaps that is why they are friends’
Collins Dictionary (doesn't mention conjunction, lists is as "pronoun")
for or because of which: there is no reason why he shouldn't come.
Yet on the Collins Learner's Dictionary it lists it both as pronoun and conjunction
2. conjunction You use why at the beginning of a clause in which you talk about the reasons for something.
He still could not throw any further light on why the elevator could have become jammed.
Experts wonder why the U.S. government is not taking similarly strong actions against AIDS in this country.
- I can't understand why they don't want us.
3.pronoun You use why to introduce a relative clause after the word 'reason'.
- There's a reason why women don't read this stuff; it's not funny.
- Unless you're ill, there's no reason why you can't get those 15 minutes of walking in daily.
So in all examples where Collins lists it as a pronoun we have "why" coming directly after the word "reason". Oxford Living Dictionaries doesn't list it as a pronoun, but a "relative adverb".
I'm wondering why in these two particular dictionaries (Collins and Oxford Living Dictionaries) we don't find "why" listed as conjunctions? Are they not viewed as such in modern grammar?
Also, I've been warned in the past not to place my trust in dictionaries when recognising word categories or functions, which is another reason why I'm asking this.