How do they not have any leverage? How exactly does McD's operate if every person decides to boycott the job for better, more livable alternatives? McDonald's would eventually have to adjust if they wanted to stay in business.
The reason these workers and all unskilled workers even got into this type of position, where they are being paid less than what everyone knows they can even survive on, is by having the exact mentality you speak of: that they are useless and replaceable so just take whatever you can get and shut up. But if McDonald's didn't need the labor at all, they wouldn't even hire these people. So the workers absolutely do have leverage, it is just now that they are trying to use it.
But strangely, people are mad at them for it, and meanwhile support large corporations for using all the leverage they have to literally squeeze workers to the bare minimum that they can pay