Can "why" be a conjunction?

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)
why
conjunction
2 : for which : on account of which
know the reason why you did it

American-Heritage Dictionary (yes, conjunction)
why
conj. 1. The reason, cause, or purpose for which:
I know why you left.

Random House Kernerman Webster's Dictionary (yes, conjunction)
conj.
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.

Oxford Living Dictionaries (conjunction not mentioned, listed as "relative adverb")
relative adverb

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")
pron
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 didn't find "why" anywhere in the Wikipedia Conjunction article, but did find it in the Wikipedia Conjunctive adverb article.

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.

Solution 1:

The word why can obviously be used as a conjunction, as your examples clearly indicate.

The problem is with the dictionaries.

To see why, consider this Wikipedia article on Natural Language Processing. In the original “rules based” approach to language processing, the analysis of a speech fragment depended on the fragment conforming to an expression that could be generated from a grammar. The association of a given word with a part of speech would therefore determine the expressions that could be generated.

However, the “rules based” approach has been largely superseded by the “statistical” approach, partly because the rules themselves became complex and unwieldy.

But my real point is this: Dictionaries are written by human beings with a fundamentaly limited rules-based approach to language processing. As the human user attempts to apply the dictionary rules (parts of speech, etc.), he or she inevitably runs up against the same problems as the computer programmer trying to implement similar rules in their code. The rules printed in the dictionary are simply not powerful enough to deal with the range of expressions found in real life.

From the statistical point of view, why is found in multiple roles. However, the dictionary writer, aiming at readability instead of completeness, inevitably has to leave some of these out.

It is worth noting that even a simple natural language processing program like Siri (or the Office Online spelling assistant) has already analyzed more sentences than you or I could read in a thousand lifetimes. Their “dictionaries” and “grammar manuals” go far beyond what can be found in Collins or the Oxford.

So in practice, the occurrence of why in the role of a conjunction means that the part-of-speech attribute conjunction can be applied to the word why.

I suppose we could go further and argue about whether the assignment of an attribute means that something actually “is” something, with a digression into deep epistimology, social constructions of meaning, and that kind of thing. However, if we accept that the statistical concept of natural language processing has been validated through its practical application, even for the sake of argument, then we should be willing to accept the assignment of attributes by the programs that have analyzed the largest number of examples, your own examples counting among them.