Nowadays it is clear that people dress significantly less formal than they did in the past. I often see in movies, pictures, and other meidums that people would always be wearing an outfit that would be considered formal today. Practicaly everyone would also be wearing a hat of sorts. Today people wear things like sweatpants, t-shirts, tank tops, and short shorts out in public on a daily basis. If you ask the average person why they are wearing something like that they'll say "because it is comfortable" but it is certainly not presentable. What happened?
Please let me know if I should further clarify what i'm thinking as this topic greatly intrigues me. I would also like to know how I can begin to study this topic further.
There is always more to be said, but you might be interested in my answer to this question:
While the question itself focuses on hats, I mainly discussed the casualization of clothing in general.
I can’t comment on sweat pants, however my understanding is that the rise of the tshirt came about through the US military particularly the navy and the marine corps in WWII.
The US navy was the first of the armed forces to adopt the tshirt as an undergarment around the time of the First World War. However it wasn’t until the Second World War and the following years that it’s popularity took off.
Although at first perceived as an undergarment the humble tshirt was standard issue for nearly all branches of the US armed force in WWII. With the navy and the Marine Corp predominantly based in the Pacific theatre it became a hugely popular item of kit the the hot humid climate amongst the thousand of drafted and enlisted servicemen.
Following the end of the war it continued to be a popular fashion item once the veterans returned home.
This article goes into more detail: https://www.uso.org/stories/1974-how-the-u-s-military-made-the-t-shirt-the-most-popular-garment-in-the-world