[csydas]

Many of the apartments aren't designed well for such a set up, or have other strict noise rules, concerns over water problems and maintenance costs, or just simply the landlord doesn't like the idea of the units having the machines as even with more modern washers with good locks/seals, people still manage to misuse them and cause water damage. Or they just don't want to manage the maintenance for such units.

It's also possible that there's an expectation that you always have a washer/dryer combo, whereas having just the washing unit isn't uncommon from what I've seen in the EU/other parts of the world and then just having an air drying rack for drying. I was a little surprised when I first moved abroad at not having a dryer (and annoyed admittedly), but I got used to it pretty easily and just adjusted my laundry schedule. Likewise, the size may be a difference in expectation, as many people expect full sized units installed, not compact units.

[martinald]

I've lived in 17th Century buildings in the UK that have all for washing machines - I doubt the apartments there had washing machines in mind when they were built :)

[Declanomous]

You guys also put your washing machine in the kitchen. In the US the washing machine goes in a different room, which means you have to run a lot more plumbing. Combination machines are also extremely rare, and people don't really air dry clothes much, so you need space for both a washer and dryer.

Plus a unit with an in-unit washer and dryer generally costs more than a similarly sized unit, and part of that space is taken up by a washer/dryer.