You have to walk to counter each time you want something in fast food restaurant, in a regular restaurant you have waiter for that. But yeah it's his job for which he is paid, so tips still don't make sense just for doing the job, I don't get tips for doing my job, neither does my wife and most of the occupations.
To me it's not about what I have to do, but the fact that someone else is serving me for probably a pitiful wage. I'd rather they get paid well, but they don't. So I tip. Even if they do a bad job because I know everyone has bad days.
Fast food places pay at least minimum wage and these days are so desperate they often pay $15+ an hour where the minimum wage is still $7.25