I’ve never seen anyone using their furnace to run hot water. Radiator water loops are closed loops and I’m sure you wouldn’t want that water to be used for anything you’d use hot water for.

I think it’s mostly a factor of ACs weren’t historically efficient. They were all on or all off. In the more recent designs that are ultra efficient they use variable speed compressors.

They were also historically less reliable than a furnace and certainly more difficult and complicated to service.