Better question, why not?
On cooler days, wearing long sleeves helps keep your core temperature warmer, which also means your back is warmer. It's most important to keep your upper torso warm. Heat loss in your extremities has less impact on your body than heat loss near your vital organs. If your arms become cold, the rest of your upper torso may also.
Also, if a driver has to walk a lot, shorts are much more comfortable and less restricting.
Fact is, wearing long sleeve and shorts (on cooler days) end result is more production.
imo - wear shorts until temperatures gets close to freezing.