One reason you don't see too many robots like these is how difficult they are to design, plan, and manufacture. Either they're made of lots of interconnecting soft structures knitted together with glue and fasteners (each seam meaning additional labor, expense, and chances of breaking), or composed of a single skin.
I've been poking at easier ways to manufacture soft robots and think that these single skin designs have a lot of potential. I think that making robots this way could lower their cost while increasing their strength and durability. I've been calling these single skin robots plionics.
The method consists of designing your robot in CAD and working backwards from there to produce an outer mold and an inner core. Casting silicone between the mold and the core forms the robot itself and melting out the core gives you the finished product.
You can find the documentation behind a whole series of these robots here.
In this tutorial I'm going to demonstrate how to put together one of my most successful robot designs (a strange squishy creature called the Trefoil Tentacle) using a combination of 3d printing, silicone casting, an arduino, and a bit of pneumatics.