Why exactly? There's different degrees of slavery and independence, it's not black and white. Poor people don't really have a choice with their lives, they do whatever people command to them so they can just to get enough to get food.
Some black slaves in the US South got to live in the house, eat and drink with their masters, whip other slaves, and maybe even got an education. They lived in luxury compared to the field workers.
They were still slaves.
Could the slaves quit their jobs? This whole sub-thread is completely preposterous.
Can someone making just enough to get by quit their job? Technically, sure. But they lose their home, they lose their insurance, they lose their car, their children get taken from them. And now they can't find another job because they have no house and no car, and they can't get another car because they have no job and no credit. So they stay, no matter what, because quitting is a death sentence.