"datum" is pretty common in academic literature. Even more common is treating "data" as grammatically plural: e.g. "The data show..."

There are plenty of examples of plural mass nouns though, and they all have slightly different meanings than the usual sense of the word. peoples, monies, sands, waters, etc.

https://en.wikipedia.org/wiki/Mass_noun#Multiple_senses_for_...

language is complicated, and the rules aren't necessarily rationally intuitive.