While the used clothing sector provides employment to hundreds of thousands of people in less developed countries, it also damages their local textile manufacturing industry. Prices for locally produced clothes have to include both manufacturing and logistics costs, whereas the imported used clothes from Western countries are donated and can be profitably sold at much lower prices, covering only logistics.
Economies grow prosperous when there are more opportunities for economic exchange.
in the real world domestic workers don't all go on to become farmers (oops that's dumped onto by subsidized western businesses too) or some other business... some degree of protectionism has consistently worked well to the extent it's arguably necessary*. And don't take this as me defending sweatshops either ;) but a moderate degree of protectionism and public investment has very consistently allowed countries to move beyond the absolute basic "we have sweatshots and scrap out old electronics and recycle garbage" stage of development. Neoliberalism and free trade will suck away any public investment you give it the opportunity to, and engineer around any comparative advantage you can find.
But having a strong textile manufacturing industry promotes the development of other adjacent industries (e.g. dyes, chemicals) that would not be able to stand on their own initially, and can output products that are useful to more than just textile manufacturing. It easier to develop a new industry if the country already has other industries that can support its growth.
In a sense, it is like a tax on clothes, paid by the consumers, funding textile-adjacent industries, but directed by private actors rather than the government. If it pays off, other things become cheaper and potentially offset the increased cost of clothes.
In no way is being poorer the solution to not being able to afford things.
1. https://www.brookings.edu/blog/future-development/2015/04/30...
2. https://bigthink.com/technology-innovation/are-poorer-countr...
3. https://www.trade.gov/trade-guide-wto-subsidies
4. https://www.semanticscholar.org/paper/Ten-Years-of-Anti-dump...