Value of fast food "frontline workers" did not increase. It remained the same.
Those are not career jobs where people stay on forever. There is no investment in such workers that pays off in extended period of time. They come without skills, they are meant to stay for some time and leave, pretty much without skills, then get replaced with people without skills. However, those who show some loyalty and decide to stick around, they usually climb the ladder, get raises and promotions.
The value of the front line worker DID in fact increase, just like the rent, the utilities, the cost of the materials, otherwise profits would not have increased. Profits in the fast food industry are at a record high. The difference is that everyone who provided goods or services to the fast food restaurant saw an increase in what they were paid except the frontline worker. Their share of the increase in sales went to the managers and the executives.
American corporations are recording record profits. And they're doing so on the backs of their frontline workers who haven't had a real raise in 20 years.