If that were true, then we would expect the following to be true: "There are no dungeons in the real world."
But, that statement is profoundly untrue. The real world is filled with dungeons, including some truly stupendous mega-dungeons that probably vastly exceed the scope, scale, and size of anything that's been published. People actually built these things, for a real reason, and those reasons are often varied and interesting. Granted, they don't usually have anything like monsters living in them, but presumably if monsters were real, then they'd find lots of places to feel right at home in the real world.
Presumably, because he didn't trust his guards to do so.
Why are they? Mine aren't. For that matter, I'm not sure that this is even a complaint you can level against the traditional dungeon. 'Keep on the Borderlands' contains a low to moderate realism dungeon in my opinion, yet it more than passes this test.
Why is he? Mine doesn't. Mine current BBEG has four layers of contingencies in place for escaping from the PCs, including, "I'm not going to be trapped anywhere without an escape route." For that matter, I'm not sure this is a complaint you can fairly make against published dungeons. Strahd certainly doesn't have this issue, and I6: Ravenloft might be one of the two or three most iconic dungeons ever. I might not find 'Steading of the Hill Giant Chief' all that interesting as an adventure, but as a Steading the map is fairly reasonable in its design and the positioning of its inhabitants neither static nor illogical. If you aren't moving the inhabitants around, maybe the fault isn't with the dungeon designer, but the one running it?