Especially as if you had these machines, you would have to have at least 1 on-site engineer at all times (most likely two) to be able to fix / correct them when they go wrong - who would cost you $50 / hr.
McDonalds are working with a certain flow of customers mostly based on location, time, etc, not on "ability to serve".
Now, if they start throwing people off because they are just too many to serve, that's another thing...