Why do I keep finding wasps on tooth brushes?

I never heard of wasps on toothbrushes! That would give me the willies! I would say to cover your toothbrushes with a plastic bag or something, but they have to be out in the open to dry. If they stay wet, bacteria can grown on them that's probably more dangerous than a wasp.

Maybe you could find a way to cover them with cheesecloth or something, something that would let them breathe but keep wasps off them. If you can find the wasps' nest, there are easy ways of getting rid of them. Here in California we've had wasps building nests in our mailbox, and there's an aerosol spray that just kills them all more or less instantly.

Bees and wasps indoors quickly get thirsty and their sustenance is any form of nectar or sugar water.

Many toothpastes have sweeteners that can attract bees and wasps.

Indoors, I attract bees and wasps to a little honey in a saucer and just escort them outside, often doing that by hand. Paper wasps may be more attracted to very ripe fruits, here is a few paper wasps eating a crushed grape from my hand:

https://www.flickr.com/photos/32083154@N02/9767704...

Bees will even drink a little honey from your hand:

https://www.youtube.com/watch?v=LOe_X2PpNF4

And no, I have never been stung doing this.

Most toothpastes have sweeteners in them. Consider getting one of the travel holders for tooth brushes and keep your brush in that when not using it.

The sweet fragrance in toothpaste may attract them.