She just doesn't get me
blog.mashape.com
blog.mashape.com
I'd wager that an assistant service like Siri can implement a great deal of the everyday "functionality" of a human personal assistant with nothing more than precise interaction design, careful data logging, and machine learning models to tailor assistance to the individual needs of the user. That's not easy, especially considering each interaction has to be designed by hand, but it's more than possible with today's technology.
There might also be something to do with traditional gender roles--secretaries are usually cast as women, so perhaps that also raises expectations. However, I really don't know enough about gender issues to give any real insight. (Not that that ever stops me, but...)
(Not that all secretaries are women. I'm talking about stereotypes here. Also I'm not even sure that's a job that still exists.)