If you want to picture the characteristically 21st century living environment, imagine cities like Lagos and Shanghai; or Sao Paulo, New York and Cairo. Now double the population, and you have a pretty close approximation of what the world’s largest urban areas will be like by mid-century.
Now ask: How does the Army control a city of that size? How does an infantry force of thousands cut off and surround and city of 10 million? What about 20 or 30 million? The answer is that the United States can’t, at least not yet.
This is the time for mechas to make their big debut.
They could manuever in a city, step over cars and have sick robot fights. Sure, thousands would die from collateral damage, but that happens anyway. Would you rather die from the shockwave of an IED planted by insurgent forces, a tank shell that missed it's target, or a robot's rocket punch?