RTGs are generally pretty expensive due to the exotermic isotopes used. Also it needs cooling to get the needed thermal gradient for electricity production, so

RTGs are generally pretty expensive due to the exotermic isotopes used. Also it needs cooling to get the needed thermal gradient for electricity production, so just burying it into ground would likely not work. The peltier thermocouples also degrade over time. And lastly, there were some nasty radiation incidents where ex Soviet RTGs were left unattended and random scavengers managed to open the casing, getting lethal doses of radiation in some cases. I guess this could be exasperated by the small and mobile size of RTGs.