I don't want to write fiction actually

What's wrong with writing something that isn't made up? It's still a book. It's still a big challenge to write 50k words of it.

I don't mean writing a book about Random Topic of Interest Which Requires Lots of Research, such as writing an instruction manual or a guidebook to Paris.

It might be something more lighthearted. Perhaps someone really wants to write a book about different cars they really like and why they like them and such. It wouldn't be fiction, but it's still a novel. Or a factual book about something fictional, like a book about your fantasy world or a roleplaying game you've created. Same goes for biographies/memoirs and any sort of book that deals with memories. If I want to write a book about my cat, it wouldn't be fiction (unless I started making stuff up, of course), so it wouldn't "count" for NaNo according to the attitude of people in the forums and the FAQ... but isn't the challenge to come up with a continous (not necessarily coherent) text of at least 50k words in a month's time? (Continuous might be the wrong word, but you know what I mean.)

A text of at least 50k words in 30 days = a book/novel/whatever = big challenge = what NaNoWriMo is about.

I've seen in the FAQ and other threads the saying "If you call it a novel, we call it a novel", which is great. So if I write a book that _I_ call a novel, but which isn't fiction, it'll still be a novel according to that saying. And then the whole thing doesn't make any sense:
"Here's a novel I wrote about my mother's childhood."
"It's not fiction."
"Perhaps not, but it's a novel if you ask me."

Not that it really matters, as such, as no one verifies the texts and say "this is not fiction!!" - so who would know anyway? And if you're writing it for your own sake, to prove that you DO have it in you to write a novel, all types of novels should be allowed, as long as you start from scratch. Why say "you can't write that" to people who have no interest in writing fiction but who want to participate in NaNo because it's a great writing challenge? I've never written 50k words in a month (at least not comprising of the same story), fictional or non-fictional. I care about how many words about the same kind of "topic" (storyline) I can come up with, as that's my challenge. I don't care about the storyline being fictional or real, as I will be just as happy and feel just as accomplished to reach 50k regardless of if it's fiction or not.

"YAY!!! I wrote a novel of 50k words about my mother's childhood! Woohoo, I made it!"
"It's not fiction."
"I don't care, I wrote a novel of 50k words in a month! I did it! Yay me! I can write books!"

If there's no one to verify the topic of the book, why actively discourage people from writing what they care about just because it's not made up, when the challenge of writing it is still the same?