There's very little incentive for the USPS to deliver foreign packages in a timely fashion. They probably get "pennies on the dollar" to do so. So, they probably sit until there's space on the truck to move them. Or they just sit because whatever, maybe it'll stop people from ordering as much overseas?
I had a package sit for so long the tracking # stopped working (i.e. it was 2-3 months). Then, out of the blue, it showed up one day. That was at the height of the pandemic last year, though.