Why can you reuse a tenga egg?

So I got a tenga egg, cause it was discounted, I found another discount code, I just wanted to try it and if I liked it, I might get a proper fleshlight.

It says the tenga egg is one use, unless you use a condom. I don’t mind using one, but if I don’t use one and just clean it throughly with water and stuff, why can’t I use it again?