Most likely because it is easy to use canned code that is readily available. If there's no need to write your own layout, why bother? But if there is some need for customization, then make your own layout and pass that through instead. Here's a link to a tutorial for an SMS app that uses a custom layout for rows in a ListView:
http://adilsoomro.blogspot.com/2012/12/android-listview-with-speech-bubble.html
Hope that helps!