Because its part of the vanilla mc experience. Nothing is permanent, there's no mechanic to ensure protection against it, and its part of the idea of being an open world where you can do anything. Some people choose to hide, some start groups and raid, etc. It always seemed like the intention of mc is to give a minimal system to see all the things people can do. Its cutting out a whole realm of possibility to not allow it.

With that said I hate griefing passionately so I pick nongriefing servers.