Let's imagine the OS thought so, too, and had programs require implementation of both UIs, one for machines which is hard to look through by humans, and one for humans which is automatically presented in GUI form, meaning its hard to control and it's hard to parse the information it's presenting in a bitmap window (IOW an unautomatable interface). Now, I see 2 reasons to prefer the scenario we have now with unix and text based communication:

1) We don't need to depend on each individual program's programmer to present every control and information consistently between the 2 interfaces.

2) Automation matches normal, manual use. Just put what you normally do on the command line in a file and you're done. There's no need to look through documentation on how to do what you so frequently do, only in a manner that you rarely do.