If someone has a question from one of those older sites, they should go ahead and ask it. But a wholesale importing of content from elsewhere is not really a desirable way to build this site.
There is a lot of ownership and careful curation that goes with vetting the content of this site. Questions imported from elsewhere would always have that air of odd, forgotten legacy content back-dated and anonymous with no owners or real-time vetting at all. If someone posts another answer or asks for some followup to one of these questions, no one will receive the notification. 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.