Cost efficiency, the same reason we have automated factories and coin-operated beverage dispensers. You don't need to pay a robot. So, if you get a robot that can do the job just as well as a human can, and which costs significantly less than a human's salary and benefits over the robot's lifetime, then getting the robot would be the wise thing to do.
We already have all sorts of devices that do this.
There is no further cost efficiency to be had by making them look human.
An automated bartender that looks like a cute girl instead of a machine has had only one function added to it, and that is the function of a plaything of one sort or another.