What makes you so sure? Slavery was universal across the human race, or practically so, until well into the second millennium AD. Nobody seriously questioned it. A bit like interest-taking now, which is why I brought it up as an example. Borrowers may complain about their monthly payments, but they don't seriously imagine themselves the victims of injustice qua borrowers.
We imagine that "if I lived back then, I'd be against it", but we forget that the past is a foreign country. People thought and acted quite differently.