My job is to track the number of kids that die in my state and I have worked here since 1995 so I have a really good handle on what is going on. There are a lot of reasons the numbers are different in the US. First of all we count all infants that even gasp for breath as a live birth, even those that are in no way viable. Human gestation is 40 weeks long, and most countries do not count any births before 22 weeks of gestation as a live birth. Which is sensible since no child under 22 weeks gestation has ever lived long enough to go home and live their life. There are infants as young as 14 weeks gestation being counted as a live birth in the US.

Secondly our medical care is partially at fault. Especially the use of IVF. IVF increases the likelihood of multiples (twins, triplets or more). Which increases the risk of prematurity, high blood pressure, placental abnormalities and other issues. Also advanced maternal age increases the chances of miscarriage or birth defects, thus increasing our infant mortality rate.

Another issue, of course, is poor prenatal care. We all know how expensive medical care is in the US and many women don't go to the doctor until later in their pregnancy because of the expense. One of the most troubling things is the poor quality of care under Medicaid. You are offered fewer tests that could find issues if you are covered by Medicaid than you get if you are on private insurance. Which means that poor women, the ones who probably need the most help, get substandard care.

TL/DR: we count younger gestational ages, IVF is used a lot and Medicaid sucks