The problem is that (in terms of combat) you add a whole lot of complication with very little gain. Building a bipedal mech that can walk on it's own (even without enduring recoil from firing a weapon or receiving fire) is a huge engineering challenge in and of itself. Designing for combat scenarios is even more daunting. But let's say you throw 3 decades and trillions of dollars at it. What do you gain? Advantages: -Higher firing position -Possibly better performance in some terrain Disadvantages: -Much larger profile in the vertical direction, meaning you're more vulnerable from [I]all[/I] sides. -Worse performance in almost every terrain. -Loading is distributed much more compactly than a tank, meaning the odds of destroying a bridge or other structure through shear stress are much higher. -The trade-off between armor weight and movement speed will never match that of a tank. In my eyes, there's no real advantage to building giant mechs in realistic terms. I think the thing we'll see that comes closest to it will be exoskeletons.