Previously (B2C): incentivized to deliver great customer experience to make money on increased sales volume and higher-price purchases
Now (B2B): incentivized to deliver easy seller experience to make money on increasing number of sellers and sales volume
To be fair, Amazon still cares a surprisingly amount about its end customers, when it doesn't need to, as a legacy of its rabidly awesome B2C days...
... but over time the company will continue to errode and optimize itself to its current customers (sellers).
Why doesn't Amazon do more to strategically combat fakes / bad products? Because sellers literally don't care about that. It's not a priority for them, so it's not a priority for Amazon.