It's much easier to slap thick armor on a leg than on a track, though. On a tank, even an Abrams, any hit to the thread with an AT weapon is pretty much a guaranteed mobility kill. To blow out a mech's leg, you'd likely need to hit a joint, because poking a hole in the armor itself somewhere on the leg could not suffice (as long as it's not weakened too much, it'll still support the mech's weigh). Not to mention legs do provide a mobility advantage to small mechs (though anything above MW "light" category would be questionable at best), a tank can't drive through dense foilage, rubble or over barricades, while mechs could easily step over those. The real problem is that legs require an incredibly and precise fast computer just to figure out how to best move them. Humans already come with one, but replicating those capabilities is though. That's why MW uses neurohelmets, the pilot's brain controls the servos, meaning that problem is out of the way.