A lot of risks in life kill X% of the people each year, and it's human nature to ignore or minimize those risks, because folks get TIRED (fatigued) of being lectured all the time. They can hole up for a while and then they decide they want to be out and about because this thing isn't killing very many young people.
And humans also confuse anecdotes with statistics. "I don't know anyone who got it, so it's safe to party hearty."
We all rationalize our behaviors, it's not unique somehow to Americans. The Europeans are protesting against the lockdown in numbers, and protesting itself is likely to spread this.
Our neighbor the physicist is just back from Turkey and doing a two week self quarantine voluntarily. We'll be taking him dinner and groceries from time to time as we did last time. Of course, he's pretty bright, Harvard PhD in theoretical physics and all that.
We have 35,000 or so every year die in auto accidents. It's accepted as part of life, folks do what they can to reduce the figure, and it has been cut almost in half.