With the fine pitch SMT packages used in modern cellphones, if the TTL serial IO doesn't go somewhere it is probably inaccessible even if you open up the phone (some ball underneath the middle of a package with no actual access.) Presence of a serial port doesn't necessarily guarantee software to support doing anything "interesting" with the data that arrives on that port.
And the goal is a phone that you don't have to open up just to talk to, and a phone that will do something useful when you talk to it on its serial port.