Cars are getting larger generation after generation. See the Golf just for one example. There are few exceptions to the rule.
Second, cars of ten, twenty years ago were easier to park also because you could see where they ended. With many modern cars and their awkward shapes and tiny rear windows you can barely judge their size from inside.
And third, even if "normal" cars (sizewise) might be parkable without PDC, the Model S is a behemoth!