In my opinion it's very often a matter of expectations.
When a lot of design decisions are taken by a bunch of people, from management, engineering and marketing, including a designer would simply mean having one more person to compromise with. Companies working this way tend to have designers focus more on making the product look good, thus this dribbblisation of design. On the other side, engineering has historically been expected to deal with design when building products, and nowadays a lot of engineers have learnt how to design on the field. This reinforces the view that designers are only useful to make things look good. Lastly, the tools we use in design are really easy to use, anyone with a couple hours to spare should be able to create some stuff with most of the available tools. With that, it makes deciders think that the only difference will be a matter of taste.
A lot of designers can actually do more than that, the basis of their job is usually to question things with the perspective of the user: "Does it make sense to do this? Is it really useful to have that amount of customisation? How much information do we need to see?" Most of the time the solutions are stupidly obvious, or they're chosen based of how complicated it makes the system, or how costly it can be, and that's where experts add a lot of value.
All in all, if all you need is to make stuff look good, sure a dribbble designer is probably a good fit, just don't expect them to automagically make the product usable. If you want someone to work on your experience, you should probably have your designer work on the problem instead of having them "upgrade" your solution.
Just like how the engineering field is, the design field is really diverse with people doing different things, differently.
(Wow, that was a long rant, sorry about that.)