> I really don't see any way to justify working 996 without some sort of absurdly high pay,

Many people don't just work for the immediate monetary benefit. Some like the company or see some value in working there or in that industry specifically, some want to prove themselves in a competitive environment to raise in ranks and see better chances even if the starting position is bad etc. ... Some might even like the rough working conditions because they appeal to their work ethic. Slavery would mean they have no choice but they do and they choose 996.