I'm not convinced that this list of future stars from Roseanne is disproportionately high, considering the relative popularity of the show and the number of seasons it ran. Roseanne was a pretty big show back in the day, so of course it featured a number of people who were relatively unknown at the time, but went on to greater things.
By way of example, consider 3rd Rock from the Sun. If 5 "future stars" is your criteria for "so many" then this show featured:
Bryan Cranston (note this was before Malcolm in the Middle)
Megan Mullally (before Will & Grace)
Joseph Gordon-Levitt (in a starring role, arguably this was his break-out into stardom, but it could as easily have amounted to nothing.)
And that's not counting actors who were already sort of famous (like Bruce Willis, mentioned in the comments.) Some of those from 3rd Rock include John Lithgow (of course), Wayne Knight, and Phil Hartman.
This show was popular enough at the time it aired, but still nowhere near as big as Roseanne, so if it can even get close to the same number of "future stars," then I think that says something about how not unusual this number of future greats actually is. I'm sure if you picked any other sitcom that ran 5-ish years, you could find analogous quantities of actors who went on to become household names thereafter.
TLDR: Everybody who's famous now came from somewhere before.