h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

if the plural of tooth is teeth then why are toothbrushes called toothbrushes and not teethbrushes as they are used to brush more than one tooth?

Because you can't use a teethbrush if you only have one tooth, which is discrimination.

It would seem if you only had one tooth, whether or not they're called teethbrushes or toothbrushes is rather moot.

My Grammar doesn't need a toothbrush/teethbrush. She just uses a glass of water beside her bed and a steradent tablet.

It's the way our minds naturally work. You don't call something that X's Ys a Ys-Xer, you call it a Y-Xer. (where Y is any noun and X is any transitive verb)

I'd say for the same reason hairbrushes aren't called hairsbrushes.