Isn’t that because a driver will instinctively pull left (instinct to protect their own body) when facing a head on collision in many cases? Also the rate of being thrown from the vehicle, being pierced by objects from outside the vehicle, and the risk of unsecured things (including passengers not belted in - wear your goddamn seatbelt!) flying forward from the back all being higher?

Not sure how the saying still works if those types of things are the main causes for passengers riding shotgun being statistically higher to get fatally injured