Because they are designing by committee. Samsung surely already has World-class designers on their team. However those designers don't have dictatorial power to make hard decisions and tell entire divisions of the company that they are SOL. But that is what would be required for them to put out something great.
That is why it is so much easier for startups to innovate than big companies.
Well, I see that point, however it's just hard for me to grasp - clearly Samsung (like many other hardware manufacturers) have to acknowledge that their bottleneck often is design and UX. Yet they have all the resources to change that readily available.
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.
That is why it is so much easier for startups to innovate than big companies.