Reusing washed glass bottles for perishables like milk can be a public health risk. It's hard to get the cleaning process to the point where when the stars align for that 1 out of 100,000 bottles it still gets sterilized.
Until I see statistics proving the opposite I don't believe that. Washing bottles hot enough to kill germs really is not hard and we used to reuse lots of glass bottles.
Getting glass hot enough to kill microbes isn't hard, but it does take a lot of energy. Microbes will attach to the glass and use that as a heat sink, so just hot water isn't enough, you have to use enough to heat the glass.
It's actually tremendously hard to do that on an industrial scale. We still get listeria outbreaks even with pasteurized milk and new bottles. Adding in a source of contaimination is going to make it that much harder.
The issue isn't getting bottles hot enough to kill germs. It's doing so in a way that doesn't aerosolize the bacteria and preventing all the other ways cross contaimination can occur.
That's not the risk. The risk is introducing harmful bacteria into a bottle and then having it reproduce for some days before someone drinks it. Since you're not storing milk in the glass it's not an issue if it isn't completely sanitized.
