Seems like it would be reasonable to say “the reason why blabla etc” because the why is part of the blabla clause. I dunno what the grammatical term is, not preposition but something else. Just saying ‘the reason why’ on its own is tautological for sure.
The reason for some event
The reason behind some decision
The reason why something happened.
You’d ask why something happened, so the answer would be ‘the reason why it happened..’
You could always rearrange the sentence to do away with it though.
Plus, as I’ve demonstrated, just because a language is someone’s first language doesn’t mean they understand its grammar any better than someone who had to learn it. When you are 2 years old you learn language purely by imitating what you hear. When you are 20 and studying it from a book you learn the rules.