If the item is (looks) unopened and pristine, Amazon will usually sell it again as new.
Some switcharoos are caught, some are not.
vs
Cost of negative image (deferred via simple returns) + cost of inventory loss duo to fraud + cost of inventory loss due to employees
It's simpler to simply eat the cost of abuse.
This is a big reason I stopped purchasing most goods from Amazon.
I was tired of getting open-box merch sold as brand new.
In Washington State, Fraud has a very specific meaning, and requires me (the victim) to prove 9 elements of fraud.
These elements include proving Amazon intentionally mislead me by stating false facts, and that as a result of the fraud, I suffered damages.
As other threads have mentioned, the sheer scale at which Amazon operates means they deal with logistical issues differently than other retailers. To ensure 100% rate for returns would increase the cost to Amazon, which would either be eaten by the company, or passed along to the consumer.