I would imagine the combo of spatial reasoning and mapping plus social stimulation could be a reason. You could also argue both are regularly training reflexes and fine motor movement.

Or could be there some weird variable that's unaccounted for ? Do taxi drivers and ambulance drivers for some reason have more regular sleep patterns ? We know that is definitely helpful for Alzheimer's

Your reasons listed under a HN comment are more plausible than those listed by who worked on this subject for years. I find it funny and admiring.

Taxi drivers and ambulance drivers seem like two jobs that would have less regular sleep patterns, TBH.