Why is life expectancy of USA men today so low?

It's not.

It’s down to 75 years, a bit above Mexico’s but lower than China

We eat and drink too much garbage

We do not exercise enough

We sit too much

We work too many hours and under too much pressure

We are undersexed

We worry too damned much about trivialities and cannot turn off our anger and stress

We are horrible about getting regular preventive medical care

Our conveniences are poisoning us

Covid, drug addiction, and suicide.

Drugs/opioids

+1

Search "deaths of despair"

Life expectancy is useless metric. Our advanced medical industry can keep sick people alive forever, hooked up to machines.

The right metric is fitness tests and body measurements.

% body fat

Endurance tests: 2 mile timed runs

Strength tests: pushups, pull ups, sit-ups, squats, deadlifts, etc.

A human doesn't need regular preventive medical care. A human needs a proper diet, exercise and sleep. You only need medical care in emergences (trauma) or if you aren't eating, exercising and sleeping right.

As a late 20s man I hate work and the lifestyle it causes. Fast food, stress, inactivity. This leads me to love drugs, alcohol, and avoid sleep so I can have more free time. I don’t see myself living past 40 nor do I want to. I’m sure millions of others are in the same boat and and will suffer the same fate bringing down the statistic.

Wrong. The second leading cause of death is cancer. Early detection (which happens when people get screening as recommended) can save lives. If you’re a man over 50 and not being screened for prostate and colorectal cancer despite having access, you’re asking for trouble.

Survival of the fittest

Top leading causes of death among men in 2018 were:

Heart disease
Cancer
Unintentional injuries
Chronic lower respiratory disease
Stroke
Diabetes
Alzheimer’s disease
Suicide
Influenza and pneumonia
Chronic liver disease and cirrhosis

Most of these are related to lifestyle.

cdc.gov

From the CDC-Leading Causes of Death-Males all races and origins 2018

CDC

Current and previous listings-leading causes of deaths-males, US

Open link

If you eat right, exercise right and sleep right you can eliminate everything on that list except accidents of course. You don't need the medical business. They can't do anything anyhow. The medical and scientific root cause of all of those problems is diet, exercise and sleep.

Unintentional injuries and suicide are the top two for 0-19 and 20-44. Homicide is #4 for 20-44.

Yes, that tells you what has a bigger impact on life expectancy.

Note that "unintentional injures" = both drug overdoses and car crashes.

OP’s point is that we can’t despite “our advanced medical industry”, rather American men have an anomalously lower lifespan compared to other countries with a comparable per capita gdp and quality of medical care available.