I’m with MissAdventure (and others) - you can’t just make sweeping assertions about daughters in law without looking at your own attitudes. “Toxic” people are usually that for a reason- there are two sides to any relationship.
I only have daughters but they treat their mothers in law with respect and love , they are after all the mothers of their husbands and grandmothers of their children and in the same way my lovely sons in law show me equal affection and, yes, respect.
To go back to,your question, I think it should more accurately, read Why are bad relationships so prevalent?