>
It is, only reason people ask why is to see what your issue is just in case it's not an issue
Like, "hey, do you wanna go to a party with us?"
"I dont want to"
"Why"
"Susan will be there"
"Nah, she got arrested for smuggling cocaine into a police station and having it fall out of her asshole"
"Ah, sweet, let me get dressed" >

I don’t think this is unpopular. >

I used it this morning…my wife rejected it >

I kind of agree but I feel like that would require dictating how people are supposed to feel about the answer. You have the right to say it for sure, but people can inevitably feel offended/ ask why too. 
"Hey, I'd really like you to be there for my birthday party"
"I don't want to"
In this instance, you absolutely have the right to what ever you say, but language and communicate isn't one sided and the way we can contrive these messages can differ. I don't want to because you're tired is different to that of not wanting to because you dislike this person and don't want to attend their party. The other person in this scenario will naturally be curious as to what variant this may be. >

Who said it wasn't