Artists used to show off their capacity to paint hands. It was both a flex as well as a composition technique to balance the frame. Artists have forever mimicked each other. Right handedness dominated life and as such was much more frequently depicted to engender relatability to the subject.

It would be like having a conspiracy theory over why horrible HDR photography was so pervasive in society in the early to mid 2000s. It just was because that was the thing doing the rounds back then.