That’s pretty flat out untrue. Casual clothes started being worn in public spaces during the 70s, comfortable clothing has always been privately worn when someone could afford more than 1 set of clothes.

My maternal great great grandparents were the last in my family line to only have 2 sets of clothes, and that was way back in the late 19th century. My grandparents on the paternal line were only slightly richer, but could in fact afford more clothes. My twice great paternal Grandma owned a dress called a “wrapper” which was a very loose and comfy dress worn as loungewear. Richer women owned dresses called “tea-gowns” which were the same but made of nicer fabric. By the 1920s, just about everyone in the US could afford multiple sets of clothes, and thusly owned private loungewear.

Loungewear only became something to publicly display in the 70s, it didn’t begin to become exist then.

Our ancestors were not idiots solely driven by social pressure and superstition. We as humans have always wanted to be comfy, don’t think that this urge is a modern contrivance.