In general, being overweight is a far, far larger problem in the West, and being underweight is extremely rare outside of eating disorders like anorexia. There are people who are naturally skinny due to how their metabolism works, but they are rarely truly underweight.
[...] according to 2010 data from the Centers for Disease Control and Prevention, these are the measurements of the average American man:
Height: 5 feet 9 inches (69.3 inches) Weight: 195.5 pounds Waist circumference: 39.7 inches
https://www.cdc.gov/nchs/data/databriefs/db288.pdf
That's far more a concern than whether men need to bulk up or not. Which naturally happens as men age and their metabolism slows anyway.
Anecdotal evidence from Europe: in our industry, culturally associated with obesity, I meet roughly equal amount of obese and underweight people. And I seriously doubt those underweight people spend time in the gym.
The reason could be a tiny outlier minority.
You used the term "many".
... or me at 18. Damn I miss that metabolism.