Look at yellow lines the other way around: Consider a pass sometime, and if you decide it would be safe ask yourself why it isn't a passing zone.
Nobody uses engineers to set speed limits, so why on Earth would you assume that they're involved in selecting passing zones?
The problem with drivers is that lots of idiots think that yellow paint has magical properties and would never dream of crossing it- even though passing in double yellows won't get you any more points than doing 35 over will.