This sort of thing: https://taplanes.co.uk/
It's a bit different when the modules are part of a hotel/apartment block that's entirely built using a modular system. That's common enough in city centre hotels and motels where they're a single full-sized room and nobody can tell the difference.
If it became common I'm sure crews will switch to using a crane for the roof, just because the crane will be there to lift the bathroom in place anyway. However someone needs to start making the factory bathroom first and convince builders to use it.
1) they are heavy, the actual structural design needs to take this weight into account
2) since they have an "own" floor you either have a step to enter them or you need to have a somewhat thicker underfloor on the rest of the house
3) since they have an "own" ceiling, the internal height will be smaller than the rest of the rooms (here in Italy this is not an issue from the techincal norms as rooms are minimum 2.70 m and bathrooms and corridors can be only 2.40)
4) since there is the need of connecting the internal utilities, electricity is the lesser problem, but typically you have two or three (cold and hot + in some cases circulating) running water pipes, two (at least) drain pipes, two or more venting pipes and the radiator (or under floor) heating, you need some additional space on the outside or some (not really pleasing to the eye) boxes on one wall or in the floor (or both)
In practice the only kind of building where they make sense are new multi-storey, multi-apartment buildings and hotels, hospitals, prisons, etc., and even then usually they are not usually much cheaper, they are only much faster and easier to put up, and definitely have an advantage for "luxury" bathrooms where local workmanship capable of a high quality work are not available or cost too much.
An example, this is an Italian builder of such "pods" with experience in international projects:
https://www.eurocomponents.eu/en/portfolio
https://www.eurocomponents.eu/en/modular-bathrooms-for-inter...
I agree overall with your point though.
Usually these pods are lifted by a crane and "inserted" in the building (talking of reinforced concrete buildings) after the structure has been built (before building the outer walls).
Once they are on the floor they are moved manually on carts/wheels attached to the pod structure, so it is much easier if everything is at the same level, the lowered level of the deck (only where the pod goes) would add another complication in the moving/placing.
With the advent of underfloor heating the thickness of the screed is increased anyway (compared to the old buildings) so the "step" is not anymore a problem as it was years ago.
Under this definition of housing US housing isn't speculative either. It's only the land that goes up in value. It's just that earthquakes and tsunamis aren't as frequent and powerful here and the zoning laws are more arcane here. (as well as some other reasons related to the speed of development of these countries). Japan is one of the least afordable places to live in the world. That is in large part the reason why the population is declining. But nobody is speculating on US bathrooms.
>But nobody is speculating on US bathrooms.
I've got two words for you, man: rainhead shower. (Or "in-floor heating" for the Northeast. ...I guess that's 3 words.)
This video shows a full renovation, but if you’re only replacing a broken component then you’re looking at less work https://youtu.be/8E5lH_qppWg
I think generally the point of these is that you’re a lot less affected by skill level of those installing everything, so even really shitty apartments have good infrastructure. Million dollar homes in the US have worse plumbing than stuff I had in starter apartments in Tokyo (not universal but!).
And the boxing up means that your “wet area” is limited (Japan is generally very humid so mold is a constant worry). But I’m not an expert, and everything I’m saying might be totally off base.
These guys? (If so, then, woah, this is the first time in literally years that Google Search has successfully returned a first-result win.)
There is more than that though, as the floor needs to be designed for where the pipes and wires of the prefab bathroom come out. Not impossible, but not easy either. A lot of modern houses are adjusted on sight for where the pipes have to be as the architect doesn't check if a pipe and beam are designed to go in the same place.. (in large building the architect will check, but not in something small.)
I think there’s a massive opportunity in modular bathrooms. I’ve seen them done amazingly badly (think: the plastic cube badly perched in a corner of your Victorian seaside hotel, in which you have to crouch under the shower, while it hoses down the toilet paper), I’ve seen them done so well you wouldn’t have a clue you were in a room delivered in a box. The latter, I’ve only seen in Latvia, by a Latvian company - but I think the concept has legs.