Especially when there are no customers to have to be around, like in sweatshops and other places out of sight, it's needless. Sweatshop/factory conditions suck badly enough that they don't need yet another thing for managers to nitpick over. Heck, even wallfart used to not require uniforms, just a vest that employees could remove when going on break, but they changed that in 2007 too. One literal sweatshop I used to work at, over 110°F when I first worked there as a temp in September 2019, only changed it right before I was hired on in November so instead of being able to wear tank tops to deal with the smothering heat we had to wear their ridiculous nurse cosplay slavewear (since we worked with hospital laundry, one of their hospitals they made a contract with that year required all the wage slaves to dress as nurses) in heat stroke conditions. It's absurd.
I get that police/military and a few others makes sense to wear uniforms/armor, but in other things like factories and places where there's just work to be done without the snobbishness of “rank” to lord over each other with, it should just be a matter of letting the workers wear what they like to express themselves and deal with the weather (although really air conditioning should be legally required, rather than just coercing waivers to save bucks.)