The short answer is ... "cuz everyone likes to format the numbers differently". So rather than some elaborate "formatting" screen for the List Manager to worry about, or some series of dropbox choices to slow everyone down, we opted to leave the formatting up to the individual. At the end of the day, what you want is to be able to a) read the number, and b) call the number. And that you can do!
So, pick your format and type it in that way. The next time you go to enter it, GoToList will remember and you're good to go.
Some examples of phone number formats from around the world:
123 123 1234
And these are only for the United States. Want to see more? Go to this Wikipedia article..