People with money had their clothes made to order. People who didn’t have money often made many of their own clothes, including for quite some time after ready-to-wear became commonly available. And ready-to-wear wasn’t all the same thing made in only two versions.
There was definitely a difference between formal clothes and going-to-town clothes, as well as a difference between going-to-town clothes and clothes that might be worn to clean the house or dig ditches or dig in the garden. But jeans and shorts weren’t going to town clothes, and tshirts weren’t even shirts, they were men’s underwear. The type of clothes that were ordinarily worn when going anywhere in public look formal to our eyes, because we now ordinarily wear in public clothes that would once only have been worn when doing grubby work or on a picnic, as well as clothes that would once never have been worn as outerwear at all. It has very little to do with whether people had jeans and tees available to them – they did – and a very great deal to do with what’s socially acceptable to wear in which situations.
My mother put a dress or skirt on – and not a housedress – to do the grocery shopping. She dressed up to go to a party. Those weren’t the same clothes. For that matter, there were different levels of party clothes – still are, in some circles; but they’re a hangover from earlier years, not a new thing.