I suspect the reason is:

Close to half of all parcels entering the US under de minimis were sent from China, according to a 2023 report by the US Congressional committee on China.

There was also the thing where you could order items on eBay for $1 and they were shipped from China. I could not understand the economics of that so I did some research and found that a Chinese-American was working with/for the USPS to make that sweetheart deal. Not sure if it’s still a think but I think that started the floodgates of cheap Chinese goods into the US. This article talks about that, not sure if it was ever fixed: https://www.forbes.com/sites/wadeshepard/2017/11/05/how-the-usps-epacket-gives-postal-subsidies-to-chinese-e-commerce-merchants-to-ship-to-the-usa-cheap/