

The main reason it annoys me is because a lot of English-language recipes online are written by USAmericans and therefore use imperial units that I have to convert. I’ve also noticed that they seem to measure everything in cups even for things where it makes no sense to use volume (flour is a common one—isn’t baking supposed to be precise? Flours can have different densities, air pockets, etc). I find ounces really confusing because they can be volume or mass.
Also, imperial is not standardised, so different countries have different amounts for a cup, an ounce, a pint, etc. I guess I can check the recipe author’s country and then convert to metric. But I just find it confusing. Can’t we just all use the same units that are nice and easy to use together since they’re powers of 10?

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.