This was something Valiant did that made it unique/interesting. Various comics were set in the future or past of the same universe, so you actually saw this progression where in 4000 A.D. humans as a species were fighting aliens and malevolent robots, then eventually going out into space and so on. There were entire series based entirely in the future in this set timeline, while still having others set in the modern era.
Whereas both DC and Marvel only seem interested in exploring the futures of their universes in one-shot adventures, as "possible futures" etc. Even series like X-Force eventually became just a "possible future" with Cable acting as a modern-day hero when he teamed up with Deadpool and created his utopia, etc. There is no historical set time-line for the future.
The only exception I can think of to this is Batman Beyond which, now that it has been brought into line with the New 52 (sort of, I think the only problem originally was Barbara Gordon being paralyzed, which has been changed, so now the only contradiction of the future world of BB might be things that are considered non-canon like The Dark Knight Returns). On the other hand, it might still be considered non-canonical by the DC higher-ups for all I know, or might be rendered non-canonical. In any case, it is only "near future", not on the same scale of the human race exploring other worlds or what have you.
Basically the big two want their universes to exist in stasis, possibly because fans complain if they see "the future" of their various heroes and it's not what they want, or this is just the perception of the people in charge of them.
EDIT: Oh, wait, I remembered the Legion of Superheroes. I think that's a good example of a future-set comic that is very much a mainstream DC property.