Well in a way they *do*.
Down about 20 feet or so below ground the ground temperature is the average of the annual temperature range. There is so much ground all around that it simply doesn't heat up or cool down significantly. In the UK you can pretty much guarantee around 9°C ±0.5°C all year round, and unless you go so deep that the geothermal energy starts to play a part, that's what you will get.