There are a couple other villains in the piece that force employers to respond to increases in wages that exceed a worker's productivity. If he did hire such workers, he would earn lower profits. Soon, investors would abandon him and put their money where returns are higher.
There's another villain -- the customer. If the employer retained workers whose wages exceeded their productivity, to cover his costs he would have to charge you and me higher product or service prices. I don't know about you, but I prefer lower prices to higher prices, and I'd switch my patronage to those firms who adjusted to the higher labor cost.
Congress can easily mandate higher wages, but they cannot mandate higher worker productivity or that employers hire a particular worker in the first place. Those of us who truly care about the welfare of low-skilled workers should focus our energies on helping them to become more productive, and a good start would be to do something about the rotten education that many receive.