I've thought about this a lot as a huge sweater who's thinking about quitting cycling outdoors due to sweat (chamois chafing, spraying sweat on others, etc.). I'm a sprightly 75kg in pretty good fitness but have a really high normal body temperature.
I think the extra airflow from their speed has something to due with it, but that doesn't explain on climbs. I'm amazed how their hair is completely dry during/right after rides while mine is soaked.
Agree with everyone regarding genetics
, maybe pros got more into cycling partly due to their low sweat genes?