There's a difference between weight gain strategies to build muscle and eating because you're "underweight." People not working out and eating normally will be less bulky than people who work out and eat comparatively more. But they won't be "underweight."
Everything is a "thing for a reason".
The reason could be a tiny outlier minority.
You used the term "many".
I used the term "many" specifically because I do not have the stats. Not a majority, but a significant number of people, that struggle to gain weight.
If you have no data to back up your claim you should not make that claim. The word "many" is a borderline weasel word.
Sure it's a thing for a reason... the reason is guys who are into body lifting that was an easy source of calories and protein. It has nothing to do with your narrative.