Why can guys lift like 100kg/220lbs at the gym but struggle to pick up their girlfriend who weighs 54kg / 120lbs?

Why is this always the case. Guys lift tons of weights at the gym but when it comes to lifting their girlfriend or the tv or a package or the couch which weighs less than what they lift at the gym, they always struggle. Are the muscles there for decoration or what?
Why can guys lift like 100kg/220lbs at the gym but struggle to pick up their girlfriend who weighs 54kg / 120lbs?
Post Opinion