Quite simply, they are brick stupid..
They don't understand that the wind resistance increases exponentially the faster you go. Their bike goes just fine at 10 mph in second gear at 2,250 rpms... So 3,500 at 70 mph is ok.
They don't understand that all engines have a sweet spot.
They don't understand that you need to hold the next gear a little longer than the last one as you accelerate.
They don't understand they are lugging their engine riding in top gear around town at 50mph.
They don't understand that the manufacturers put TALL gearing on the bike to pass emissions.
They can't comprehend that an engine can return better fuel economy and a longer service life if ridden at speed at a higher rpm. It goes against everything they "know".
That is all.