The problem is that the most advanced part of the engine assumes a static grid.
It is possible to get a brick to "plant" itself visually to the client but remain a temp brick to the server, invisibly movable but not colliding.
From that I can assume that buildable vehicles would be completely possible, *but* it would require an update that gives vehicles an optional brick grid of it's own and that the client is able to draw that grid.
I don't know how the engine works, but I can assume that it would take a long time to add for little gain, so it would only be implemented if there was nothing else to do.