Cardboard recycling seems fairly successful (and perhaps Amazon boxes are already returned directly to Amazon?), but it does seem like there has to be a better way.

Amazon eventually realized that boxed returns were inefficient and implemented unboxed returns via UPS stores.

It seems like unboxed delivery to a store or locker would also be more efficient, but for customers it's less convenient than front-door delivery.

Perhaps delivery workers could also collect reusable Amazon boxes while making deliveries.

Cardboard refuse is environmentally benign. It's wood fibre (and a few other bits) which break down in nature, usually within a few weeks or months at most.

Plastic refuse persists for hundreds to thousands of years, and is itself biologically active, often interfering with endocrine or other bodily signalling functions, if not other long-term chronic poisoning, or sheer physical blockage.

Cardboard sourcing of course has environmental impacts through deforestation, monoculture, and land-use impacts. That's another story.