Don't confuse CGI with "made on computers". In CGI, everything, including the backgrounds, is rendered as a set of digital objects. This is not how The Simpsons or Family Guy are animated; they are "hand drawn", but the drawings are now done directly on computers rather than on acetate cels. (Also, most of the vehicles on Family Guy are 3D models; this makes it much easier to move and rotate them.) I doubt that South Park is CGI, especially as it still takes quite a bit of time to create the CGI images themselves (it still takes years to make a CGI movie) and the show has a quick turnaround time in order to get current event references into the show.