With slaves you have all of the costs of maintaining and housing and feeding them and giving them basic medical care. Letting them die is a huge burden of cost and loss of investment. You have to cover at least their bare subsistence.

With Proletarians you don't even have to do that. You can pay less than subsistence as the reserve body of labor replenishes itself through reproduction. Thus you can squeeze all the value from someone then toss them aside for the next. You can let a percentage of your workers fall into health issues and die without any ability to afford care. You can let a bunch of them become evicted and houseless, and eventually die on the streets. There's another worker there to pick up where the last one left off.

This is a big reason why capitalists don't openly use mass enslavement anymore, and why the south lost the civil war. It was too expensive and inefficient compared to proletarian workforces. The slavery that does get used is subsidized by the state, so that the employers get prison laborers for cheap without any of the costs associated with maintaining them.

They want all the upsides of slaves and none of the downsides

This was the exact rationale for the Nazis' program of "extermination through work" and is why Cesaire was absolutely correct to say that at the end of capitalism lies Hitler.