It's difficult to couple hard and soft components. The difference in stiffness creates stress concentrations that lead to the interface failing. This is also why rubber pads on laptops always tend to fall off. Creating a gradual change in stiffness is possible, but is difficult to accomplish. There is also interest in soft robotics research to make robots entirely soft. The reasons for doing this is that a soft robot can potentially not injure a human being if it's entirely soft and to make robots that can squeeze through holes. I'd also like to argue that much of the interest in soft robotics today is stems from DARPA's Chemical Robots from 2008[0], a big goal of which was to make robots capable of squeezing through holes, which has obvious military applications.

[0]https://techcrunch.com/2008/07/01/darpa-throws-33m-at-biomim...