The first time I encountered a baking recipe with ounces, I looked up ounces as I didn’t know what they were. Imperial units aren’t taught in a lot of countries. The Wikipedia article said ounces could be mass or volume, and also had different values in different countries. So I was just very confused as to what the author meant.

Everybody knows what a ml is, what a gram is, etc. They have the same value everywhere. There’s no ambiguity. What is “customary” for you is a confusing system imposed on the rest of the world who want to understand what you mean.