It's shipped by air. It is so slow because of sorting, customs, paperwork. The last mile postal shipment is actually quite fast in USA.
Not an American, but I believe there's a 2006 bill that prevents USPS from selling services below the cost. Learned about it in a John Oliver's video [1], mentioned somewhere else in this thread. I'd guess that Chinese epackets are part of an international agreement, so they are exempt.
> The system was illogical.
It would be illogical if USPS had flexible costs.
But the USPS costs are more or less fixed. The mailman will go his round every day and will get his salary, even if he has little to deliver. So, you may as well have him deliver Chinese epackets, for whatever price the Chinese are willing to pay. And that's a good deal for the public. Otherwise the $1 trinket for China would be $6 (if Chinese had to pay full shipping), or $10 (if bought through a US-based reseller).
The system was illogical. Whether you want to say that US shipping is too expensive, or Chinese shipping to the US too inexpensive is unimportant.