Or there are just a lot of pre-made sets? I am curious because they are similar, but it never seems to repeat at all.
Houses are a bit of both. Some houses (such as the fancy ones) are basicay predestined in the JSON files, while many of the ordinary average ones are randomly generated on creation.
Basically most of them are randomly generated because that cuts down on repetition, but there are some hand-crafted houses that spawn very occasionally to spice things up.
On that topic, is the occasional impassable object directly adjacent to a door an intended sort of “barricade” thing, or just a random occurrence? Because I’m not sure how barricading a door with a bolted-to-the-ground console, or a plumbing-connected sink might be achieved. 
Your examples are not intended. Desk, dressers and tables however, yes. It’s suppose to give the feeling of someone trying to blockade a room. And probably failing.
I think it’s just a screw up with the random houses.
Like those “Bathroom larger than the bedroom” houses.
I used to live in a house with a downstairs bathroom that was bigger than one of the upstairs bedroom. Of course, the bathroom was oversized for use by the previous occupant who was handicapped and the mentioned bedroom was a guest room, but still.
Well not that its relevant, I’m just bored while waiting for the most recent version of cata to compile.