The earthbound equivalent would be strapping each chassis to the back of a dedicated solar panel and having the panel double as a giant heat sink. The problem is that doesn't work on the surface due to (at least) rain, the day/night cycle, and the cost of real estate.

That's not right. This works a couple orders of magnitude better on the ground than on space (unless your computers run at several hundred °C).

The reason people don't do it here is because it's too expensive.

Isn't a solar panel going to be a poor heatsink, though? It's flat, and thus has relatively small surface area compared to its size.