The article mentions a lot of the reasons. Our building codes, especially the double stairway requirement, eliminate a lot of reasonable layouts and have downstream effects on the economics of apartment buildings. Another reason that isn't mentioned in the article is that there are different ideals for family housing, especially in suburban areas like Walnut Creek. Family apartments just aren't as desirable.
While building codes are different, those are not different enough to explain the difference. I've seen plenty of people try to propose layouts for apartment buildings that would meet the U.S. requirements and be much cheaper, but no one builds them, even though they should be legal.
The real problem is families in the US don't desire apartments and so no one builds apartments that families would desire to live in, thus perpetuating the problem.
Family apartments could be really desirable, but nobody builds them because of the building codes so people don't look for them. The point that a single-family home costs 1.5 million dollars is very applicable here, as the monthly payment on that loan is cripplingly expensive or totally out of reach. A 4-bedroom apartment for a family of 4 would be extremely desirable in that market.
I think it's really a simple, cultural bias. Most wanting 3+ bedrooms and having the means are also programmed to be inclined to want detached houses with a yard.
Also, in California, "apartment" means short-term leasing and is usually full of young singles, couples, divorcees, and retirees. It is often a big building with many studio, 1, and 2 bedroom units. A similar multi-dwelling structure with individual unit ownership is referred to here as a condominium (condo). These often have slightly larger units since they are targeting the home-buying family rather than the itinerant renter.
In SF Bay Area suburbs like Walnut Creek, there are also neighborhoods with town-homes. These are like condos in ownership structure, but often only have 2-3 units per building, wrapped with a small private yard maintained by the owners. And these lots are in turn surrounded by larger common grounds, which are maintained jointly by the HOA for the whole neighborhood.
In the US the nuclear family is usually two parents and two children. The parents share one bedroom and each child gets one.
In Asia I would guess families make room for their older relatives which would mean an extra bedroom is needed. Immigrants to the US typically do that but I think it's uncommon among the rank and file Americans.
The article actually explains it: with 2 staircases requirement the cheapest[+] option is to have the internal shared corridor and thus you get just one external wall for an apartment in general (unless it's in a corner where it gets two). If you arrange a bunch of bedrooms along the wall you end up with an internal corridor, wasting space, and making your whole apartment looking like a sleeper train car.
Houses, on the other hand, have 4 external walls at the very least. Not to mention most 4+ bedroom houses are 2+ stories as well.
[+] It might be also required to have staircases at decent distance, as a second staircase going through the same stack of apartments is not of much use as it's likely to be blocked by fire at the same time as the other one.
Mostly US apartments don't go beyond a certain size, after that you need a house or maybe a townhouse. Double maybe a condo, but those usually don't go bigger than apartments either.
Apartments are for dense housing, big apartments aren't dense, therefore look elsewhere for your large apartment, I guess. You could have 4 tiny bedrooms, I guess, but that's weird too.
It's nice to have an office/guest room, but if you're raising a family that needs three bedrooms in apartments, you're compromising.
Because there’s a heavy bias for people who have use for 4+ bedrooms to also not want to live in an apartment building.
The reasons vary, but I have lived in Boston/Cambridge area for 40 years and can’t think of literally any of my friends who raised a kid past the age of 3 in an apartment, despite many of them (including me) enjoying the apartment life while young and single. But, literally as soon as we could [barely] afford to, we bought a house and only then added to the family.
Smaller units rent for more per square foot in general, so if the building project is limited by gross-floor-area ratios, developing it into more, smaller units produces more rent than fewer, larger units.
There’s also a pretty strong effect in the US that selects towards raising 4+ person families in a house (whether rented or owner-occupied) if at all possible, so a 4 BR apartment is likely to be occupied by the (relatively) rare tenant who prefers to live in an apartment, but more likely one who is forced to by lack of option. I suspect this effect is much smaller in Asia.
In the US, when you start to get to planning to have a larger family, especially the size of family that would necessitate 4 bedrooms (multiple children, or maybe having a parent live with you), most people expect to move out of an apartment building into a house anyways (maybe detached single-family home, maybe duplex, maybe condo). So there's just not the demand for 4-bedroom apartments or even 3-bedroom apartments to a large degree.
