Why is the mass of leaf measured in grams?

Answer 1

Because mass is measured in grams, milligrams, kilograms, etc. and grams is the sensible choice considering kilograms would be a ridiculously tiny number.

Also, grams is the "base" measurement, like a unit of 1. Milligrams would look more like a normal number, but grams compares it to something that weighs, well, one gram, which is a lot more known to us. (ie we can lift an object and say it's about a gram, but it's a lot harder to tell if something is one milligram because you would barely even realize you're lifting something.)