I do not know anyone who wears 'absolutely anything from their wardrobe' to work each day. My experience is that everyone has a private 'work uniform', whether or not there is an externally imposed one.
For example, if like a previous poster I was expected to work in jeans and a T-shirt, I would need to go out and buy some, and I would keep those for work. Although I do have a jeans and a T shirt or two in my cupboard, they are of the 'slobbing at home and doing the gardening' variety, and wouldn't meet the requirements of even the most casual workplace. Equally when moving from an office job to a primary teaching job, I acquired a new 'uniform'.
Since teens don't in general earn their own money to buy clothes, but are more susceptible than adults to peer pressure in the matter of brands etc, it is probably easier for them to have an 'externally imposed' rather than a 'personally imposed' uniform. Certainly I, as a full scholarship holder at a rather posh girls' school, was far more comfortable when wearing uniform than when we had to wear non-uniform - the stark difference between my family's income and that which was the norm for the school was invisible in [secondhand] uniform but all too obvious in home clothes.
If I genuinely believed that all teens, given the option of non-uniform, would wear a cheap and comfortable, non-branded, durable, decent and practical set of clothing instead, I wouldn't have a problem. As the downsides of non-uniform in terms of peer pressure and expense FOR ME outweight any issues around blazers vs sweatshirts, I'm happy to keep the uniform option.