I've talked with some of those crews anonymously on Space Chat Roulette. it's not that great of a secret...Here's the embarrassing truth - they constructed these mega ships around the cargo they carry, and plain forgot to leave an access way to it, which is why you see these mega ships cruising around, but never landing, docking, or meeting up with any other ships, stations, installations or facilities. They're staffed and crewed by the most dedicated telepresence operators, with the highest of security clearances, to keep The Great Secret.