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?