There’s been quite a lot written about the loss of open space in the US, if you’re curious. Huge tracts of land have been developed into housing and suburbia, and that leaves quite a lot less for horses.
There are also lots of areas in the US with poor soil conditions, or very dry/very wet weather conditions. Keeping horses outside full time may be challenging in a desert or swamp, or may be really damaging to the land when the soil is fragile.
When we lived in the Midwest, I was BLOWN AWAY with how good the soil was, and how well grass grew, and how quickly everything recovered after horses just beat the crap out of the paddock. There’s definitely a reason why that region is farmed the way it is! Other areas–the dry land prairie of the west, or the rocky hills here–just cannot take that abuse without turning to eroded, muddy “dry” lot.
Yeah, as a society, we are probably more risk averse. Some people just don’t want their horses at risk of getting hurt. Some don’t want to deal with a muddy horse, or a 30 minute hunt for their horse in a large field, after dark, after driving 60 minutes to their boarding barn, on top of a 60 hour work week.
Most of us would love to have a beautiful grassy field out the back door, and horses that live out (yay no stall cleaning!) but that sort of bucolic vista isn’t the reality in many places 