>The heat source in this case is cars, which are literally radiators. Cars are actively driving through the space and even at rest they are radiating heat.

Not that much. The "warmth" is also true in garages or levels where there seldom are cars coming and going. One every while. Even when the garage is closed.

The other reasons stated (the thermal behavior of the structure) are more promising.

Ventilation is a major factor, at least on hot days.

How does the outside air temperature affect the temperature of underground parking lots? Are they as hot or less hot during the winter?

CO is deadly; air must be recycled very quickly. All else being equal, it seems like ventilation would bring the facility to outside ambient temperature quickly.

http://www.mobilecalibrationservices.com/Ventilation_for_enc...

"For enclosed parking facilities, ANSI/ ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality specifies a fixed ventilation rate of below 7.62 L/s·m2 (1.5 cfm/ft2) of gross floor area.2 Therefore, a ventilation flow of about 11.25 air changes per hour is required for garages with 2.5 m (8 ft) ceiling height. However, some of the model code authorities specify an air change rate of four to six air changes per hour. ..."

The heat is substantial, although it may not be enough to explain it here. My own personal garage, at least, has an elevated temperature for many hours after I park. You have a ~400 pound block of steel heated up enough to boil water, that's going to contain quite a lot of heat.

Don't think about the car's movements inside the garage. Think about the fact that this big lump of metal has been running around for half an hour getting hot before it decides to use the parking garage to cool off. This is a massive amount of heat that persists for hours.