It takes a lot of time to educate onesself, and a lot of people don't like to learn. Education is a great response to the people who do, but it's also not usually an issue for them. It's much easier for people to just buy something based on review or testimonials.
Myself, I don't care about the scientific aspects of an amp or DAC. I don't care about the THD, crosstalk, etc. I don't care if it sounds "warm" "lush" or "smooth". I'm only interested in the engineering, and the engineering decisions that went into making it. I'm interested in the challenges they faced and their ways of solving them.
As a result I ended up with a DIY kit that was cheap, fun to build, and extremely interesting. It sounds good to me, and that is all I want.
But many people don't know what they want. It's much easier to be told. Perhaps this is a cynical outlook, but it is an explanation that coincides with my observation. As a result it's easy to see how the current state of "snake oil" can come about.
While Sonicrevolution is right about many things, there are lots of companies that employe engineers for product design and development, not just the big ones. Larger companies will have whole teams of engineers designing and testing their products. They may even employ or contract the services on an industrial designer to pretty it up a bit. I've worked with Industrial Design students a lot and they have some great ideas, only some of which are financially viable from a design and manufacturing perspective.
My father is an Engineer, primarily focused on product design and development. He now works for Green Mountain Coffee Roasters ("Keurig") He said one reason he went in to the type of engineering he does is so that he can point to an item on a shelf and say "I made that".
Other engineers are critical to the industry but aren't so high-profile to consumers. CPTbadass designs things that many people won't really see, but thousands of lives depend on him doing his job properly. My Brother's friend works as an engineer making industrial robots, so his products most people never see, but without them some things can't be made, or would be too expensive to reach a wide audience. Such things are necessary to the industry as a whole, and to promote progress in Technology.
In some cases a company designs their own product and pays a manufacturer to produce it. In some cases they just put some additions and a logo on a product designed by another company (like the WASD V1). In other cases, another engineer is paid to design a product, a different company is paid to make it, etc. There's lots of options. This network of companies relying on each other is probably a good thing, and is certainly indicative of modern global commerce.