human nature is generally to "want things"...it is how we are wired... human nature is generally more fulfilled when we get the things we want...along the way though, the "journey" starts to increase in importance, almost to the point that it can be every bit as satisfying (or even more so), than the destination...
what does that all mean , I don't have a clue but to answer you, GA provides more challenge, more opposition, more effort, more dilegence and most important, more reward to collect... buying and collecting SA is appealing if you truly enjoy that character, have a set of standards from which to you collect (only 9.2, only white page, etc), etc...but there has to be some sort of "satisfaction" in collecting, or what is the point... GA offers that on almost every level... not to mention the history of the genre, the reminder of a simpler time, the lure of fantasy of being 10 years old in 1939 with a shinny mercury dime in your pocket...these are the things that make GA collecting "special"