Automatic transmissions

I reviewed on your website how to service an automatic transmission. It was very helpful and by the way the vehicle looked like a 2005 GMC Sierra.
I have always wondered why ALL of the fluid inside an automatic transmission is not changed when a tranny is serviced. Isn't it true that just dropping the pan and replacing the filter only replaces 2 or 3 quarts of fluid which is a small percentage of the total amount of fluid? It just seems like with changing engine oil, that changing all of the transmission fluid would be the way to go. Thanks.