Life expectancy is a weighted average (no pun intended), and so it's unusually sensitive to outliers. People who die early drag the average down much more than people who live close to the mean life expectancy. The biggest premature killers of Americans are obesity, drugs, car accidents, and suicide. Anything that addresses one of those causes of death has an outsize effect on life expectancy. There are 100M+ obese Americans. There are about 100,000 overdose deaths per year. Obesity, while not as lethal as drugs or suicide, afflicts 1000x as many patients, and so a treatment for it can have a large effect on the numbers.
Which, I recognize is a pretty privileged way of putting it—people struggle with weight, mental health, and drugs, and those are real struggles that shouldn’t be ignored. I just also want to see where things are developing on the upper-bound for reasonably plausible lifespans.