Unless those questions are deemed off topic on those other sites, we have no cause to remove content from another site and the reputation/ownership that goes with it.
Besides, the wholesale importing of content from elsewhere is not really a great way to build a new site. Questions imported from elsewhere would likely be orphaned if the author does not have an account here; the tags probably wouldn't match; the posting might be different, and anonymous content always has that odd, back-dated feel of long-forgotten questions that no one really cares about any longer. If someone were to post another answer to one of those old threads, or even ask for a bit of followup, no one would receive the notification as they do now.
Essentially, we would be loading this site up with a lot of questions asked and answered a long time ago… without imparting any of the benefits of reputation, ownership, or experience into the community that is supposed to take care of it.
That's why we don't do it.