For any product there is always a buyer. Accordingly, there is a corresponding demand for products, such as coinplate, even at declared prices. For people like you, who are able to make an coinplate's equivalent with the tools and materials at hand from the nearest shop, coinplate seems overpriced. But it should be borne in mind that not everyone has such skills and the desire to actually make seed phrase saver like coinplate by hands. It seems to me that the coinplate and analogues of other competitors are aimed at such buyers. I almost forgot to point out that they are also ready to expect delivery (buying locally is incomparably faster) and neglecting the safety of personal data, such as payment details (if you pay not in cryptocurrency) and delivery address. Buying locally leaves no "trace" that you bought the coinplate and respectively, you are the crypto owner.
Yeah, don't get me wrong here. I'm not criticising that they're doing this, I'm just suggesting those that are willing will find easier ways of going about this, with less security risks, and more convenience. The security risks aren't all that much as has already been discussed, for example using PO boxes for both security, and privacy is probably wise.
As suggested above their markup isn't crazy, and does save you some of the effort yourself, so definitely the local metal works route is definitely for the Do It Yourself (DIY) type of people.