For starters basic math. There are something like 6 billion plus humans on this Earth. If 100 million Americans were killed. 100 million West Europeans, 100 million Russians, 100 million Chinese, and 100 million from other countries that would total 500 million killed. Or less than one tenth of the global population.
I'll also note that if 100 million Americans were killed that would leave the surviving population of the U.S. at about what it was when I was growing up in the 1970s.
You and others forget how enormous the world population is or even how large the U.S. is for that matter.
And don't start prattling about the mythical "nuclear winter". Not even Carl Sagan (who proposed it) believed it by the time he died (read one of his biographies)
And given that the U.S. hasn't tested a nuclear weapon in 30 years, I'm pretty certain that you have no direct experience with their effects.
correction: Earth's population is now almost 8 billion.