We tend to underestimate the human interaction, however it's far harder to refuse a "would you like fries with that?" type of question coming from a human, rather than from a stupid interface on which we'll tap "Skip" as an automatic gesture and without regrets.
Talking with another human is also good when you're undecided about what to buy. Of course, it's not like McDonald's is a varied restaurant, when in fact they are famous for having those 15 dishes taste the same wherever you go, but there's still choice involved when picking one of those burgers. And think of how in restaurants, even with a detailed menu with pictures, etc. people still ask the waiter "what do you recommend?".
So yes, you can automate a cashier, but this means that the customer <-> McDonald’s interaction also gets automated in that process, this being a doubly edged sword and my guess is that it's not the customer that loses.
Oh, and the irony of this automation trend is that in the end there won't be enough people left to pay for McDonald’s shitty burgers, unless we progress towards some socialist society with minimal income and so on, in which case McDonald's raison d'être will cease to exist.