There are lots of duplicate products with slight naming style variations. No one seems to have solved grouping products together where the same UPC hasn’t been provided from each seller. Would require building a parser/ some AI that understands aspects of the title: brand, line, size etc

UPC wont do much. There's somewhat of a scam where sellers will reuse old discontinued products or buy upc codes from ebay in order to get around any filtering Amazon has.

Ideally Amazon should require the UPC to be fully registered and verified instead of just taking whatever a seller enters at face value.

eBay have been trying to enforce UPC/EAN for some years now but don't seem to be making a lot of progress either.

Amazon have also made the burden of proof for getting changes to a listing made so high that it isn't really worth the time to point out the issues to them unless it is a product that sells well, so the long tail is a mess.

I think this isn't a technical problem.

Ebay has the same challenge, and similar solution (Ebay uses dropdowns, while Amazon uses small clickable boxes), but I see the problem much less on Ebay, so I think Ebay is simply policing sales more than Amazon does.