The root cause is that quite a few years ago the consortium of national postal providers which setup agreements on who pays for which parts of international shipping decided that China was classed as a developing country and given subsidized rates by other countries. This wasn't a big deal when the agreement was made because it was before the days of pervasive online commerce, and there just wasn't that much sent from China to other countries via the postal service.
I encourage everyone interested in this to checkout this podcast episode from NPR's Planet Money: https://www.npr.org/sections/money/2018/08/01/634737852/epis...
> The reason for this price gap, Jayme claims, is a secretive group of postal policymakers that meets every four years to fix international shipping rates. A kind of postal illuminati.
> Today's episode: A conspiracy theory that's actually real. How the decisions made behind closed doors make it cheap to buy things from across the world, but are also distorting the global economy.