What you describe, according to my experience with corporate design, is the result of the poor level of creativity in applying design concepts and design methodologies.
Applying the product design process principles for shipping products require a lot of creativity. As a designer (UX, UI, IxD etc), you should be able to involve and to engage no-designers people, and this is a great exercise in terms of creativity.
Just think how to present your design solution to the head of product or even worst to the board members. Very often these are no-designer people with no-design culture with solid design opinions and, last but not least, questionable taste.
You need a lot of creativity to present your ideas/concept/deliverables in a sexy and a straightforward way.
From the other hand, I do believe, that involving creative agencies during the design process is always a good idea. Very often the design team is overbooked with the product roadmap and cannot be focused on important topics like UI collection (icon-sets, colour palettes and image repository) and/or new interaction design patterns.
Regarding the acquisition of web agencies by corporations, I believe that for some managers this can be a shortcut to inject design people into the product development process. It can be a good choice (from the budget POV), especially if the agency has a background about projects and products.