It could be because:
1) Fake product issue is not as widespread (per user, seller or per transaction)
2) Fake product cases are usually resolved in a satisfactory enough way to dissuade the consumer/seller from seeking alternatives
3) The annoyance of getting a fake product is outweighed by other conveniences offered
If any of the above is true, well, there is not a real problem for Amazon to address.
Of course, the assumption is that there are theoretical alternatives to Amazon for getting stuff you need, minus the convenience perhaps, but with higher degree of trust in product quality.