It’s because fuel economy is heavily driven by vehicle weight, since start from a stop kills efficiency. Cruise effeciency is more about aerodynamics than weight (ask anyone who’s ever towed anything - you can really feel the drag above 45mph).
And oddly enough, today’s cars aren’t really significantly lighter than 30 or 40 years ago. We’ve just moved the weight from the frame/body setup to unitized body/frame (lighter but safer… And cheaper to manufacture), more safety systems (airbags/computers) and things like heated seats, etc.
Today’s 4+ seat SUV often weighs as much as a 1970’s 4+ seat station wagon…but with less space inside.