But in the US it is frequent that the price per weight (or per volume) is not displayed consistently in the same units, for example $/oz for some items versus $/lb, and it's much harder to do conversions than with the metric system.
In my experience this difference is decided by what a "normal" amount of the good tends to be. For example, €/kg is usually used for meat, flour etc, but for something like spices or herbs a price of €/kg would seem silly since you rarely buy, and it's rarely sold in, a kilo. Still, a luxury of the metric system is that even if it's listed as €/100g, all you have to do is multiply by ten (aka move the decimal one step).