Continuous contact versus repeated near-falls
A wheeled or tracked robot is, at almost every moment, in stable and continuous contact with the ground. Its wheels form a wide, unchanging support base, and the control problem is largely about steering and speed, not about whether the robot stays upright at all. Gravity is a solved problem for a car, it just sits there being pulled straight down into a base that was never going anywhere.
A legged robot has no such luxury. Every step involves lifting a foot off the ground, and the instant that foot leaves, it contributes zero support. Whatever weight it was carrying has to be picked up by the remaining feet, and for a biped mid stride, that can mean a single foot supporting the entire robot. Locomotion on legs is therefore not a variation on driving, it is a repeating sequence of brief, controlled near-falls, where the robot deliberately destabilizes itself to move forward and then has to catch itself before gravity finishes the job.