It's because you only ever translate but never speak or synthesize latin exept in a few church circles where it is or was used as Lingua Franca (such as depicted in Conclave last year). I understand the original post to be about the profound difference this makes in acquiring a language intuitively.
I'm skeptical how much speaking/synthesizing the language matters if you only care about reading.
I can read German moderately well (can get through newspaper articles pretty easily, and novels with some effort), but I have very little ability to synthesize it (it'd take me quite a lot of effort to construct a sentence in writing, and I can't really speak at all). But the lack of ability to produce the language doesn't seem to negatively impact my reading ability.
And this is the case for most scholars of ancient languages besides Latin and Ancient Greek. While those two big ones get the occasional translation of a modern work like Harry Potter or The Hobbit, nobody is writing new works in Sumerian or Middle Egyptian, although reading existing works is what these scholars do.
