They aren't really fully built, it's just that all the bare architecture is there.
Which means they haven't built the ship interiors. Even if they have gotten as far as grayboxing the interiors, they're not complete, which is why I sarcastically noted that they have not added them yet.
Which leads me into my next point, judging how many issues Star Citizen has tackled in the past few years, I wonder if Frontier really understands the scope of building fully modeled ship interiors. The Anaconda, the largest playable ship at roughly 280 meters, is larger than pretty much every Star Citizen ship. Currently the largest ship nearing completion in Star Citizen is the Idris, a frigate class ship that is about 239 meters long. CIG has not only taken years developing the interior (which I could post, by the way), but has a team working on it that is larger than the entire Frontier team combined. They even estimate it will take roughly 50-70 players or NPCs to crew effectively.
Given this, I am once again going to say that I doubt the ship's in Frontier will ever be available to walk in, and if they are, Frontier is going to half ass the hell out of it.