Well, if it were just one piece, then it wouldn't be plural, would it? So, it would just be biscotto?
I guess Italian morphology didn't creep in to English.
That is why you don't eat spaghetto, or raviolo. For spaghetti, I can see it, because, nobody eats just one piece of the pasta. There always is plenty of them.
But what about ravioli? What if it were just one? Well, nobody serves it just one either. But what if it were really really large?
Then it wouldn't be ravioli, nor raviolo for that matter. It would be a hot pocket.
It's funny how words behave. Take dumplings, for example. What is the origin of that? Is it dump? Well, we have duck and duckling. Maybe, it is indeed dump, but since you don't want to make big dumps in your pot, you make them smaller, thus, instead of dump, you make dumpling.
Does that make sense?