This happens everywhere, not only in the USA, though the degree of disparity varies from country to country.
Gender Ratio - Our World in Data provides a lot of information on this topic.
The two most relevant details are:
- Women, on average, live longer than men. This means that all else being equal, we would expect females to account for slightly more than half of the total population.
- sex ratios at birth are not equal. In all countries, there are more male than female births.
For most countries, there are around 105 males per 100 female births.
the sex ratio at conception is equal: there is no difference in the number of males and females conceived. For births to be consistently male-biased, there must be gender differences in the probability of miscarriage through pregnancy.
That is, females are less likely than males to survive until birth, while after birth, males have a lower life expectancy than females.
While dangerous situations (e.g. employment, risky behaviour, and war) contribute to the male death rate, the difference becomes most significance at higher ages where these factors don't apply:
Deliberate sex-selection may slightly contribute to the antenatal rates (as can be seen during certain periods in some countries), but is not the general cause: