As a former fast food employee, I promise you that it's much better having to clean an additional machine once a day and occasionally attach a new box of syrup than it is to have to constantly refill drinks for customers, especially considering how annoying the general public are to serve
Why are soda combos gross but alcohol combos are not?
People will spend $15 on a cocktail with olives and shit in it, but no we gotta draw the line at people putting a splash of Mr. Pibb into their Coke Zero...
(I love self-serve fountain drinks and I'll die on this hill, motherfuckers.)