Surprisingly often I see guys in the gym, which have a totally jacked/trained upper body but untrained legs.

The first times I saw a guy like this, I assumed perhaps there are some medical reasons to not train the legs, but by now I saw too many guys like this, even sometimes in a group, for explaining this.

These guys know obviously how to train and I just wonder why you would invest so much time/energy in your upper body w/o doing a minimum for your legs. It also seems not too functional; what good is your upper body strength in real life if you cannot use it for lifting something heavy, because your legs are too fragile? Further, it looks very unbalanced to my eyes and usually the stars/models have a more balanced look, so I don’t even understand where these people get the inspiration/trend.