I know a relative adverb "why" can be followed by a clause and make the clause a noun phrase. It can play a role as an object or a subject in another sentence.
I know the reason why she is still single.
Sometimes, the antecedent is okay to be dropped, if readers can guess there has to be the antecedent "the reason" with the context.
I know (the reason) why she is still single.
Likewise, the under-written sentence is possible.
The reason why she is still single is because she is ugly.
However no native speaker says like this:
(The reason) Why she is still single is because she is ugly.
Wrong??? Why? Is there any specific reason?
567. Reason In an informal style, why/that is often left out. The reason she doesn't like me is that I make her nervous. 『Some people consider it incorrect to use a because-clause as a complement after reason (as in Sorry I'm late - the reason is because I overslept).』
the place where ..the day when ..the reason why ... 『The place, the day or the reason can be dropped in an informal style, especially in the middle of a sentence.... Spain's where we're going this year. Why I'm here is to talk about my plans. (More formal: The reason why I'm here is...)』