This is for euphony and pronunciation. "Cashed out AIG stock" is difficult to pronounce (you have the "t" at the end of "out," which you have to pronounce to a stop and only then proceed to the next word) and doesn't sound very good coming off the tongue, and so the additional word "of" was added. This makes it easier to pronounce and it also sounds better - the "of" enables the words to roll off the tongue easier (the "t" at the end of "out" can be slurred together with the "o" in "of", giving "outuv" or "ouduv").
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.