Most robots are rigid. Rigid is easy to design, easy to construct, easy to calibrate, and more reliable for all of those dull, dirty, and dangerous tasks that robots excel at. When robots make fundamental structural compromises to rigidity, they do it in complicated ways, like with series elastic actuators or hydraulics. It's worth it, though, because adding squishiness can make robots both more capable and safer to be around through passive compliance.
Taking this concept to the extreme has resulted in some incredibly squishy robots, including soft robots that can walk, and other soft robots that can roll. But in both of these cases, embracing squishy properties means giving up rigidity. MIT has been working on a structure for a robot that offers both: squishy when you want it, and rigidity when you don't.