It's not. If you were at the restaurant, you'd tip the server. If you were home, you'd tip the deliverer. If you picked it up yourself, why then you could tip yourself :)
> you're using to tipping at a restaurant and never questioned it
Now that's a bold assumption :) How about, tipping at a restaurant is part of the culture, and many people do so begrudgingly?
Dissecting this meme reveals that people are comfortable tipping for the act of waiting on them (the so-called service). The person who cooks and the person who bundles supposedly serve the institution, and so they should be paid by the institution.
Or so it would seem. The waiter and the driver also ultimately work for the institution, so one could argue that they, too, should be paid adequately by the institution. Tips should be reserved for exceptional service to you beyond what they're expected to perform for the institution.
So if the cashier at the takeout place made you happy and you wanted to tip him/her for that... you could knock yourself out. It's worthy of tipping because you felt like it.