Cybersled

What prevents the design of a super fast, lightweight tank?  It would rely on speed and manueverability, lack armor, and use rocket propelled grenades as weaponry.  It would rely on thrusters and propulsion instead of the standard tank track.  Could such as design exist and still have reliable fuel efficiency? 

What about wheeled tank, but one that can move sideways or diagonal, like the spherical wheels in Dominion Tank Police?  The tread system seems merely "adequate."  Why not have a Batman Returns style wheeled tank, basically a dune buggy/ 4 wheeler for combat which would outmanuever conventional tanks and still have weaponry that can disable a tank.