I'm assuming you're talking about terms like Proto-Germanic and Proto-Uralic?

These protolanguages are not called that because they're not yet "real languages", but because they are hypothetical, reconstructed, or theoretical tongues which Germanic and Uralic languages share as a common ancestor. For instance, suppose we had no knowledge that Latin existed and texts from before 1000 CE were increasingly rare, until we had maybe a few fragments with a couple words from 1 CE. We could look at all the languages that have developed from it and deduce that at some point there was one language from which they evolved. We might call that "proto-Romantic."

The Linguistics of Mayan Writing ed. Soren Wichmann