You can rent a truck for a day when you need it and save a lot of money and emissions.
Here, you have someone who might move their single horse from summer pasture to winter pasture, and as a result, thinks that they "have" to have a HD3500 "dually".
Speed limits. Americans generally refuse to slow down when towing. In most of western Europe, the speed limit is 20mph lower (give or take) when towing. In order to tow at a high speeds while maintaining stability, you need a long wheelbase and mass.
Liability and fear of lawsuits. My 2017 VW Sportwagen is rated to tow 2000lbs in the EU. It isn't rated to tow AT ALL in the US. There are no mechanical differences that would impact the tow rating. If a consumer wants a vehicle rated to tow, they literally can't use a mid-size sedan or wagon in the US (despite the exact same vehicle being able to tow a small travel trailer, horse, or small powerboat without problem).
In practice, a vehicle's safe towing capacity is usually not limited by the engine or drivetrain, but by the ability to stop the trailer safely. Brakes on the trailer itself help, but the big problem is that hard braking shifts more weight forward onto the hitch (trailer dive) and tends to lever the front wheels of the tow vehicle up, reducing their braking ability and in extreme cases eliminates your ability to steer while braking. A weight-distributing hitch counteracts this, but in the US light passenger vehicles are seldom rated for use with such a hitch. Towing at a lower speed also somewhat reduces this danger, by quadratically reducing the kinetic energy you need to shed in an emergency stop.
This in combination with beefier rear frames on euro car models (and no truck sales this would cannibalize) results in a higher towing capacity rating. It is much more common to have a utility trailer for the 3-4 times a year you would need it.
In reality people go up to 60-65mph with a decent trailer, that is about 100km/h.
Think of the stereotypes that plywood falling out of the bed of a 2010 Tacoma evokes vs the stereotypes that plywood strapped to the roof of a 2010 Crown Victoria evokes.
100 mph is… not likely though.
And unless you're using it more frequently than that, only a fuel efficient medium / small one makes sense.
(Possible exception: infrequent long-haul towing for extended trips)
Obligatory Money Mustache article:
https://www.mrmoneymustache.com/2015/04/28/what-does-your-wo...