The German case you cite is separate from the English one. In Germanic languages in general, there never was any third-person imperative. But when German borrowed the T-V distinction from neighboring languages, the second person plural (Ihr) took on a polite singular meaning. And later, when this stopped being polite enough, the third person plural (Sie) took over that role. But German had no third person imperative, so the subjunctive was used instead. Bitte rufen Sie ihn is syntactically a subjunctive, and thus the Sie isn't optional (since German isn't pro-drop), unlike in geh [Du] weg.
In English, however, and in informal German (using the original second-person pronouns Du and Ihr instead of Sie), the subject pronoun is usually implicit. This is by far the most common case cross-linguistically, at least in my experience.
By "implicit" I mean that the pronoun still there, but it doesn't appear on the surface. In a language like Ancient Greek, which marks person on the verb even in the imperative, this implicit pronoun still triggers second-person marking. And in English (and several other languages), the implicit pronoun still triggers reflexives: "go let yourself out", not *"go let you out". This would only be expected if there were an invisible subject pronoun there, C-commanding the reflexive.
(There are, in fact, certain constructions that look like imperatives but don't have this implicit subject! My introductory linguistics professor would use "bless you" as his only example of this, but "damn you" and many other expletives fit also. See McCawley's paper under the pseudonym "Quang Phúc Đông" for the details.)
The subject-verb inversion here is particular to Germanic, and doesn't quite match any other construction in the language. For example, English negative imperatives use "do"-support even when the verb is "be": "Don't [you] be late!" This doesn't happen in normal indicative sentences: *"He doesn't be late". So the best I can say for this is, it's a special construction used only for imperatives.