Oh sure, with moderate experience with a platform, one can usually obtain fairly decent accuracy with working through a build. But I've found it unreasonable to expect newbies to the platform to achieve anywhere near the same level of accuracy at the same cost. And I bet even with moderate experience, the build cost of the rifles above is a lot more than the $399 quoted in the original post.
Why would someone want to try and build a low end AR in that range?
Now I agree at least some experience in the platform is needed to build a decent value AR.
If you want uber cheap, reliable etc watch PSA's sales and snap a lower to an upper and add a sight system.
In my case, I've done a purchased AR, sold it, then went the above mentioned PSA route. I actually like the cheap PSA better than the mid level AR I had prior. But there are some things I don't like. The worst being the cheap stock and handguard.
So I decided to build a value build. Not the cheapest I could find but the best value for the money. I'm documenting it in a thread. But the gist is to build a better gun and learn how to get into the guts. So going this route I've spent about $100 more than a blemish PSA lower goes for on their regular sales. But for that I get a much better stock, a better pistol grip and an ALG trigger upgrade over the PSA mil spec trigger. And I now know how to assemble a lower.
For the upper I need to get a few tools, then the parts. I could have picked up a PSA upper close to what I wanted, but the twist rate was different. And one of the objectives of the project is to put it together myself. I expect when all is said and done the finished upper minus optics will wind up being between $300 and $400. So I'll be about $600 for a good quality AR. Sure it's $200 more than a bare bones AR but when you compare parts it seems like a great improvement to me.
Beyond that I may be mistaken but the BGC and barrel are the most critical parts of the upper as long as the upper in in spec. So how much to spend on those items depends on what you're doing.
For most people who only casually shoot IMHO a cheap AR is great. But as one learns more, shoots more, and starts to get competitive then a few more dollars spent wisely can really get you a nice rifle for not a lot of money. And yes the sky's the limit if you want but staying restrained those of us who have other obligations can still have nice ARs