Maybe they like to show off their big muscle of upper body.
For me, it is important thing to do like upper body. I use drumbell for all workout for now. I used to workout at gym. I made some dumb mistake by skip leg sometimes before. When I first lift weight in high school, I made so many mistakes like not allow body to rest after exercise like bench press everyday. Ignore my teacher advice, why I can’t bench press every day cause body need rest for one day from lifting weight.
I guess you work out before right ?