Er, I haven't been in either a car or a cross-channel ferry in at least 30 years, but when did I say anyone did have to? I presume you have quoted the wrong person and didn't mean to aim this at me?
Because car users have the brains of elderflowers and will sleep all the way across in a crusty shell of their own ignorance and vomit. Whereas you might walk about a bit, ergo cause more damage to the ferry's valuable diamond-strung silk flooring.