For one, because servicing such large ships requires a different kind of economy. You need a shipyard for tugs, then there is the fact that constructing these ships is going to take a very long time, because your factories will have 12k or something like that, production, while your shipyard will have 1k.
It is an economic and industrial question, rather than efficiency. Do you engage your military hard and limited assets, or your economy? It's that kind of thing.