That’s the problem: it often doesn’t. I’d have an air-to-water heat pump heating my house today if the quotes I got were remotely cost-competitive with a replacement gas boiler. A payback of “never” or “not within the projected lifespan of the equipment”, coupled with far fewer people who can service the equipment and far less likelihood that any part you need will be right on the truck, means that a lot of houses, including my own, have a gas boiler installed and maybe will electrify in 20 years if it’s competitive then.
I’d have even paid a small premium ($3-6 thousand? Sure! But not well into five figures) to go air-to-water, but not 2-2.5x as much to buy into a possibly hard-to-service (due to lack of experienced techs with non-standard equipment, not because the equipment itself is a problem).
The UK is so far ahead of the US in air-to-water that it’s crazy to think we share a planet.