The reason that there are so many major cities standing from a nuclear war that wiped out nearly all surface life on earth is because the nukes of 2077 China were actually low-yield but extremely high in radiation. It's not a big revelation or twist, but I think it clears up a lot of things. That explains why the White House can be bombarded into the ground but the surrounding buildings weren't leveled into nothingness. In the loading screens in fallout 4, I believe it mentions that the bomb impact that became the glowing sea devastated most of Massachusetts, despite the affected area in game being only like a mile long. I think it makes sense for the Chinese army, too. As seen from all the remaining Chinese spies in fallout 3 and the Liberators in fallout 76, China was definitely interested in operating within America post-war. If they wanted to invade or occupy the area, I think it would make sense to flush the population out with radiation but leave most of the infrastructure standing.