A lot of central AC installations in the USA were added to the house some time after it was built, and the heating ducts were already there, so now they are used as the AC ducts too.
I am curious about the history of residential HVAC in the US.
However, most homes built in this area were built central ducted heating systems since the 1950s-1960s, when there was a massive building boom. Most older homes have had central heating added as a retrofit decades ago -- even old Victorian homes. So when people upgrade to AC today, it's very common to just use the same ducts.
It's not easy to find a house with a ductless mini split AC system around here. Most homes have a single-zone central system that cools the entire house.