There's no reason. They could find the server if they poured enough effort into it. The entire safety of Tor and its hidden services depends on there being too much traffic and too many nodes to isolate any given stream and track it.The US government -- perhaps uniquely since America has ended up basically controlling 75% of the internet -- has the power to do that. It's my opinion that nobody cares enough to bother. Perhaps I should say "yet," or perhaps they never will. Who's to say.