A case costs very little, and offers a lot of advantages. The major advantage is that you can choose between them, for aesthetic, functional, or protection purposes. You need no special knowledge to choose one. If you want one and don't want to search, you can go to a shop and look at the shelf of case options, think for a minute, and pick one. The tablet works fine without a case, and isn't going to shatter immediately (although it's a tablet, so it's by no means well-constructed).
Meanwhile, there are reasons not to implement everyone's desired feature into the device itself. I don't want a rubberized phone. In my experience, it doesn't add much protection and does add a lot of pointless volume. It helps the device stay put and not slip, but I have a metal device that already doesn't slip very much. Meanwhile, I do want a case that will protect the device from damage should I accidentally drop it, so I bought one of those. Whatever additional feature you want, you can get the case that does that. There is a difference between making the device resilient enough not to break (every manufacturer should do that) and implementing a feature desired by a small subset of users and forcing it on everybody (no thank you).