Because we haven’t seen one in a while.
An anecdote I ran into on reddit recently (here):

I was at a dog park after dark the other day, and I spent a good two minutes 'dancing' with a border collie. I'd jump right, and he'd jump over in front of me. We never touched each other, just mirrored each other's movements. It was fun.

What's black and white and well-bred all over?
Looks more like a Karelian bear dog to me.
Good god, you’re right, it does.
I’ve never seen or even heard of one before. I wonder if it really is?