They don't inspect every return - that would make the already ropey economics of allowing returns very unviable - and it's often hard to detect return fraud because the fraudsters are returning an object that's nearly the same as the real thing, just used in ways that maybe aren't obvious given a casual inspection. Or they've raided it for parts and the object looks the same but the internals are gone.

It's not only Amazon that has this problem btw. Lots of online stores do. Return fraud is so prevalent that you should expect to this to become more common. A few bad apples ruin it for the rest of us.

Payment details aren't enough to reliably establish identity in many cases. If fighting this stuff were easy they'd have done it already, they aren't idiots.