They are definitely tracking suppliers, but it is not as easy as you make it seem.
Suppliers may not be using 100% fakes, making them much harder to catch. Customers may also be a significant part of the problem, due to Amazon's return policy. Customers can easily claim a fake, then ship a fake item back, blaming the supplier.
There's no way to know for sure without Amazon themselves testing the products, which would be costly. Most items do not have unique serial numbers, and even if they did, opening boxes to confirm matching serial numbers would likely be cost prohibitive.
Suppliers may not be using 100% fakes, making them much harder to catch. Customers may also be a significant part of the problem, due to Amazon's return policy. Customers can easily claim a fake, then ship a fake item back, blaming the supplier.
There's no way to know for sure without Amazon themselves testing the products, which would be costly. Most items do not have unique serial numbers, and even if they did, opening boxes to confirm matching serial numbers would likely be cost prohibitive.