Why do English dubs have a unique dialect? Something I noticed while watching a number of English dubs is that characters talk... the best way I can describe it is as a dialect. The scripts and voice acting are done by native (mostly American?) English speakers, but use odd vocabulary and turns of phrase that sound like part of its own dialect. It is difficult to describe, since it sounds nothing like any real English dialect I am familiar with.

For example, the English dub of Devilman on Netflix uses the word "human" (and at one point "mortal") instead of "person" or "man" in English sentences where those would be expected instead. The phrase "go to hell, you mortals!" comes across as distinctly stilted and unnatural to my ear (as a native General American English speaker).

Is this an actual phenomenon or am I imagining it? If it is real, why does it exist?
Adapting the Japanese script of a show for an English dub goes through a few different steps and throughout these steps there is plenty of opportunity for the original script to be altered/adapted/interpreted in a way that results in the dub to be noticeably different from the original, but can also still be far away from ordinary english.

First the script is translated. In language, there are many cases where there is no direct translation or where a word can be interpreted in multiple different ways which could lead to a case like in your question. Also, there are many cultural words and references where even after translation, most english speaking viewers would simply not get it which is why it is necessary to have the translations adapted and re-scripted.

In adaption and re-scripting, writers attempt to make the translation flow naturally when spoken in english while keeping in mind the voice actors and the time it takes them to say it and the time allowed for said lines in the animation itself and also ensuring to mention all the essential plot points. This process allows for plenty of artistic interpretation and I think that is really the answer to your question. This dialect you notice is just how the writers adapted the translated script... an unnatural dialect could be due to artistic choice, an attempt to convey something that doesn't really exist in english or it may even just be bad writing. I would say however, that more often than not the unique dialect you notice is not so much unique, as it is Japanese idioms being kept in the english dub. Some examples are "obento", "shiritori" and adding suffixes like "-chan' which to someone unfamiliar to Japanese things, may seem at first like an odd or unique dialect.

For your specific example I would say it was an artistic choice, but I would also say it is not unnatural, and it even fits well with the theme of the show. The word mortal may not be very common in your everyday conversation but it is very common in dramatic dialogue about gods, angels, demons and the battle between good and evil. There are ample famous books, movies, quotes, poems where humans are called mortals and the setting of those books and movies will usually be similar to that of Devilman. (Edit: If this was not the case, and from your comment it seems as though it isn't, then it would be because of one of the above mentioned reasons. Possibly just bad writing?)