Everyone has an opinion on this topic and individual life experiences/challenges make people opinionated/sensitive to the stories of others.
While the original poster's experience is an n=1, there is truth that the majority of Americans do not have the suggested amount of physical activity. That plus the interaction of level of physical activity (or lack there of) plus each person's average caloric intake can lead to weight gain.
Here are the mid 2010's guidelines developed by the American college of sports medicine
If adults are not getting around 150mins of moderate aerobic activity per week with some strength training, they are not meeting the nationally approved guidelines.
A 2020 study data suggested that only 20% of American adults meet those guidelines.
This is not 150mins of hard running. This is 30mins of a stiff walk 5 days a week. or biking.
Its hard to work in exercise with work and family. That being said, a huge portion of US adults do not do this. And at last measured over 40% of America was obese.
Im not saying people do not struggle to meet this. But a lot of America over eats, some over drink, and most are not doing any physical activity per week.