You're not storing the heat that's underground, it's just that the ground is a huge thing with massive heat storage capacity that doesn't notice weather as much as above ground does.

If above ground ranges from 0-100 degrees F, underground likely ranges from like 64-68 degrees F, which makes it really energetically "cheap" to get to your preferred temperature range, heating to idk 70 at most and cooling to probably not even 65.