Immediately after the events of the story, China took over the world.
The Tei Jin had natural hides so strong that nothing save attacking their eye was able to do anything to them, this going double for their foreheads. Now, think about how many Tei Jin corpses were littered all across that capital city. Probably more than actual living people there. The Chinese would have a near endless supply of invincible armor to outfit their soldiers and with very little production costs due to not needing to gather and mine the materials. In the feudal era, that would make them an unbeatable foe for any nation in the world at the time. Why wouldn't they invade the entire known world?- Logistics. Feeding, maintaining, etc. an army isn't easy.
- Also, not all sorts of skin are suitable for crafting armor out of. It could be too difficult to work with, too stiff after death, impossible to keep from rotting, etc.
- Not to mention, how can you skin the carcass of a creature whose hide is specifically shown to be impervious to your best military blades?
The film takes place within the canon of the MonsterVerse.
The Tao Tei are in fact one of the thousands of species flourishing within the hollow earth, and like Skull Island the Jade Mountain was one of the areas where it opened to the surface. They didn't arrive from outer space via the meteor, the meteor strike simply disturbed them and opened up the mountain for them to escape.