Notebook cover quandary

There is something I have been wondering.

Various manufacturers regularly put out new notebooks with specialty covers either as a yearly special or a limited edition product.

My question is, why don't they start making covers instead/as well?

Hobonichi, Midori, Kokuyo and some others make basic covers but none make decorated paper, plastic, vinyl or fabric covers that can be moved between notebooks.

It would be nice to be able to get a fabric cover printed with something like Van Gogh's Starry Night.