They were. Owners rented them away.
Whereas spare slaves who could be rented away weren't a problem to get rid off, but another opportunity to profit.
Definitely a way to mitigate down time/lack of profitability, and one that has roots in historical practice. Still, I'd argue it's much easier/more efficient from a factory owner's perspective to not even have to worry about rental; this also has roots in historical practice.
Not only does a slave owner have to find someone who wants to rent their slave at a profitable price, the owner has to worry about the conditions their expensive, fragile capital is working in (and if the renter damages their rented slave, the owner has to take the renter to court or otherwise extract damages).
Wage labor that can be hired/fired on the spot does away with both concerns: owners don't even have to hire the old wage workers, who can die, be damaged, or otherwise move in the meantime. There is extra worth for the factory owner in that they are no longer responsible for the wage worker care or employment once fired.
Ultimately, I don't think slaves are prohibitively expensive, and should slavery have remained legal for longer, I'm a firm believer there would have been ever more extensive industrial usage of slaves. My post was simply expanding on and providing concrete, measurable advantages of wage workers
vis a vis chattel slave workers, which is why the South historically used both in their factories rather than slaves exclusively.