Why aren't phase-change heat pipes more common in homes?

Expense, complexity and serviceability are your main reasons.

In point of fact, the time you list as being "most useful" would be very unlikely to work well, as hot exterior temps requiring the use of A/C are often hotter than the coils on a freezer, so heat would not move then...the time you'd be able to really save on energy with heat pipes on a fridge is that part of the year when the heat pipe alone could cool the refrigerator or freezer from cold outside temps, rather than heating the house and then cooling the fridge/freezer inside the house. But this requires a bunch of complicated plumbing with unfamiliar constraints and simple "packaged units" that require only a power outlet, and perhaps a waterline for an icemaker tend to win in the marketplace.

If you want to get fridge/freezer heat outside and you have adequate budget, commercial units requiring a refrigeration tech to run lines between an outside compressor unit and the inside cooling unit are available. They are very rarely seen in houses due to the cost relative to a packaged unit.

The same factors affect other schemes such as using refrigeration waste heat to pre-heat water.

So, you could certainly try to apply heat pipes in your house, but that's going to require that you become comfortable designing and constructing them, or that you have so much money to hire people to do that, that the energy savings make no financial difference to you. If constructing them, you will almost certainly need a vacuum pump, and you'll need plumbing skills at the "no vacuum leaks" level.

I recommend The Tubular Thermosiphon: Variations on a Theme by GSH Lock as a good introduction to the subject if you pursue the build-your-own path. It's not too expensive as a used book.