"It’s a box full of parts any consumer can buy off the shelf."
The trick is that many (most?) of us could buy those parts, but would not know what to do with them, even if we were provided schematics. That is why we have to pay more than what the sum of parts would cost.
Another thing is design/aesthetics of the product. Many streamers look better than home-brewed items.
How much is all that worth to anyone is another discussion.