I would imagine the manufacturing of them these days could be handled fairly easily by 3D printers, no? I don't know how much printers themselves vary in degrees of quality, but I'm sure there are some which could come close to the original injection-molded carts.
According to what I read on smspower.org (they are trying to make new cartridges for homebrew), 3D printing is way too expensive: it needs too much material, the plastic filaments are expensive, and the tolerance margins are not good enough to make both sides of a shell fit nicely. Plus it's apparently difficult to make pretty round corners.
Think this has been talked about before, but I feel that just by existing a repro is marked as a repro. It's just a bootleg if you are making and selling games that area already available in a region.
I consider the Repro makers the same way as how we have various publishers being able to use texts fallen into public domain. Some do fancy books with hard covers, illustrations and commentaries, others do small paperback with cheap paper that you can read on your commute without worrying about the object. They also have different prices, depending of what you are expecting in your edition.
Save for the important distinction that video games are not public domain yet, their original publisher will never reprint them, ever. I think it's only fair that other publishers take their chance at publishing the original game with new fancy art or mimicking the original release. The only scummy part of that practice is that of property of said art. It's not public domain yet, yet repro makers don't seek approval from the author nor give them a slice of their earning. But they don't see a dime of the money we spend on second-hand market either.