My position has always been that they do not exist under RAW except as magical items. And sure GM might add them. Or not. The game definitely needs to be designed based on the actual rules though, not on someone's houserules.
Perhaps it did. And that's why druid limitation is written as prohibiting certain things rather than listing things that is allowed. (Like you suggested it should have.) Then it is easier to add things later. But they never did. In any case, they must have been aware that under the rules they wrote, druid will generally will end up with a lower AC than, say clerics. And they must have taken (or at least should have taken) that into account while designing the rest of the class. Now it is another matter whether D&D is so tightly balanced game to begin with , that such minute difference is meaningful. Arguably not.
Nevertheless, the game was written with a limit on druid armours, you're not entitled to have half-plate on your druid. Having that would be a nice optional extra, and if you don't get it you have not unfairly been deprived of anything.