It's human nature and it's how you take advantage of people, especially the meek.

Small indignities, constantly layered one on top of the other, each individually small enough that their lack of moral fortitude or need for validation outweighs the immediate impulse to defend themselves, but in aggregate you crush their will under the weight of them all.

They rely on a lack of retribution too, because everyone just rebuffs them when they lie and cheat and gives in once they move on, but without forcing them to pay any actual price for it. They get to attack with impunity waiting for a moment of weakness, so they just keep trying the same lie a hundred times, waiting for the one time everyone who knows better is too tired or busy to refute them and they sucker a few more rubes.