The external team sticks around for 2 years (or whatever) until they've completed their earn-out, and then they bolt.
Think about it like this: it takes the design team six months to figure out who to talk to. By then, the product team is locking down the next version of the product. The design team spends the next year prototyping beautiful features for v.Next. v.Next rolls around, compromises are made, the design team is inevitably broken up in a re-org, and, by the time they're ready to actually contribute, their two years are up, and they split.
That is why it is so much easier for startups to innovate than big companies.
On the other hand, the longer I think about it the more I come to the conclusion that pleasing and intuitive design just isn't a major focus for those brands, and maybe not even for their customers: After all it's not something you can put as a bullet point or spec on the back of the packaging.