It does happen occasionally, there was a post here where someone’s Mini 7 had an actual gouge out of it where the charger port is (thankfully Apple exchanged it without issue, though I believe they had a long wait for a new unit.)
I wonder whether imperfections like yours are either not noticed because of the speed they are working at, or are assumed to be things that will wipe off. If a customer is buying a brand new device it’s not unreasonable to expect it not to have a cosmetic issue out of the box - it would impact the price if you tried to sell it on, even though you haven’t caused it. Though in reality you will get marks in time - I keep my iPad in a case and it has a few marks on, as did the iPad before that. I was only aware of causing one of them, and didn’t notice any marks out of the box on either iPad.
Would they have better QC on a higher end device? People are more likely to accept a minor blemish on something that cost a few hundred rather than a thousand, I’d say.
If it bothers you then the only option is to buy from someone who offers no quibble returns. Sucks if you live in a country without that, as the choice is then take the risk or go without.