"Vertigo" is the title of the well-regarded DC comics imprint, that has long been a home to the company's more ambitious, and usually creator-owned projects. It hat it's roots in DC's previous horror and fantasy line, undergoing it's more mature phase with titles like "Swamp thing", "the Doom patrol" and "Shade the changing man". Thus, their initial offerings were positioned very clearly as a bold and different continuation of the superhero-oriented properties. And yet, analyzing the starting points of the plot to some of the imprint's most famous comics acknowledges certain remarkable similarities.
Starting with "the Sandman", the series begins with the Lord of dreams reclaiming his title after a prolonged absence from his affairs. It's strange how this basic idea would repeat in the line's biggest sellers, from "Transmetropolitan" to "Fables". "Transmetropolitan", though initially published by the short-lived "Helix" imprint, begins with Spider Jerusalem, a newspaper columnist forced to reenter the field of politics after having entered a self-imposed exile. Even titles as diverse as "100 bullets" have come upon the same formula, that of characters reacquainting themselves with their previous lives, in a well developed setting. Surely these are all surface similarities, but perhaps there is a reason why "Vertigo" comics keep telling this kind of story.
And it is actually not at all exclusive to the mature readers imprint, with many superhero comics operating under the same premise. The difference is, of course, that the ongoing Marvel and DC serials are forced to tell these kinds of tales by their very nature. "Captain America" is arguably a book that starts with his reintroduction to the Marvel Universe in the pages of "the Avengers", with the character's WW2 actions in his original appearances providing the context for his modern set-up. With DC's "Justice Society of America", the first superteam introduced during the same real world conflict, their post-War status quo was defined from the more mainstream "JLA" exactly by their original adventures. The current premise of both books heavily relates to their origins, being comics that deal with previous continuity more than any other kind of theme.
With "Vertigo" comics, even the creator-owned books largely retain the similar approach, choosing to deliberately set the action at a later part of their characters' lives. Despite all the experimentation, they keep being genre books that still ignore the beginnings of their heroes' career, to concentrate on the latter days. This is true even with books like "Scalped", featuring a second generation of people living on the Rez' whose lives are defined by their parent's action. In that case, the continually referenced past events are inspired by real events relating to the civil rights movements.
At first glance, it seems strange that all of these creators would deliberately set-up a very complicated history to the characters' lives and times, before a series ultimately begins, one that directly informs their present actions. Yet, it is here where all of the imprint's philosophy comes to the core. Because, the basic idea of tweaking a common genre trope implies experimentation with the form, with the current example of "Fables", the imprint's flagship title being a modern follow-up to the well known fairy tales. In this case, the previous events all constantly referred to, subverted and built upon, are children's stories, familiar to every reader, even though the creators resist making way for having the series relate their own version of it.
Some of this may well have to do with the idea of setting a series in the past alienating a lot of readers, but such concerns could be avoided by a well-developed present day framing sequence. With "Vertigo"'s books, it seems that in order to integrate the spirit of the renowned books of films the creators seek to imitate, they all come up with a similar scenario. Because a lot of the imprint's appeal comes from applying the techniques of other media, that has experimented with the genre forms on a much higher scale than the American comics have done at that point in history, due to various limitations.
Regarding the plot, it seems that these inspirations seemingly become a part of the characters' past, which the creators then try to build upon, such as in "the Sandman" whose mythological past the book continues upon exists in the kind of real world culture that inspired it. Thus, the reader can get understand both the well-developed setting that the book starts out with, as well as the creator's own signature stylings, by getting acquainted with the ancient Greek and Roman history and culture, as well as the classical medieval literature. Similarly, "Scalped" focuses on the past that is actually a redressed as a crucial piece of the plot, based on the aforementioned real events.
Thus, it seems that by driving a clear line that abstracts the book from the fairy-tale basis that inspired it, that the "Fables" form the break with their past and are able to achieve their distinct notoriety. It seems that by starting well after the original events, these books serve to underline the differences in the creative approach. That seems to answer the question as why none of them are set in the times of their character's peak - because the more classical version already exists, and with the creators not being the original authors, they seldom seek to go back and rewrite the exact stories that inspire them. By setting "100 bullets" in modern milieu, the authors are free to think up of a new mix of the decades-old noir and detective stories that the book draws upon. At the same time, the criminal past that hangs over their characters' heads is that of a more classical time, undistinguished from the paperbacks that inspired it, which is why it's filled in only in flashbacks.
Placing an extended focus on the history that these series derive from, no matter how big a role it plays regarding the current events, would result in a period piece in which the creators could do little but pay homage and subvert the reader's expectations in small ways. By placing the story in the current time, the authors are able to apply their distinct view of the today, while conveniently placing their inspirations in the past where they belong to. It is the only way that acknowledges the literary and historical research that went into creating comics that are informed mature works on par with their other media genre peers.