Comment by energ8

From my reading of heat pumps, mismatches in energy sources and sinks can make linking things together more complicated (aka more expensive) than is commonly available.

For the pool <-> A/C case, This Old House did a segment on linking the systems: https://www.youtube.com/watch?v=J7fB8ul9dZw

Edit: chris_va has more details about why linking systems is hard: https://news.ycombinator.com/item?id=23001888