1) Because it's easier said than done to stand up to the people that have raised you. Some desi guys would never even dream of their moms being anything but angels let alone question their actions.
2) Much of this chalaki happens behind the guy's back. It's hard to decide who is wrong and who is right when you weren't there to witness the drama and one or both parties are shedding tears. It's human nature for us to believe that our immediate family members (mother, sister, brother, father) will always tell the truth. And few guys want to be stuck between the woman that brought them into this world and the one they share their beds with.
3) It's not only the women, but fathers also have the responsibility of demonstrating to their sons how a man should treat a woman. If the father himself has joined the Amma in abusing the bahu, then he's teaching the son that such behavior is an acceptable norm. And in some cases, the father is a very weak and passive person who dares not raise his zuban in front of his wife. So the abuse is carried out and nobody questions it.
4) Perhaps there is a lack of awareness about psychology. A mother has a respected place in her child's life, but that doesn't mean she's incapable of making mistakes. ALL HUMAN beings can be plagued by jealousy and insecurity at some point or the other and can allow these negative emotions to get the better of them.
5) Deep rooted societal beliefs. Our culture upholds unislamic ideas that invalidate women. These double standards have been in our society for generations. And you have two types of people. The first type of people believe that change should start with ourselves at the individual level. These type of people strive to do the right thing. The second type of people will always follow the erroneous ways of society without questioning because it's more convenient.
6) The grandmothers, mothers, chachis, khalas, phoopis have all been through their fair share of hardships in marriages. They should have some understanding and compassion and should make an effort to make things easier and better for the next generation of women. Before abusing their bahus and making unreasonable demands and having double standards, they should think that their OWN DAUGHTERS can be treated in the same way. Such a simple thought should humble them.
7) I'm not sure how much influence the media has. I enjoy Pakistani drams for the most part. But many Indian dramas, especially, still portray women in a very subservient, pati-worshipping, crap-enduring roles. Naturally, this sends out the wrong message with the wrong expectations.
Things have to change from various angles. It's a battle against human nature, family ties, and ancient societal beliefs. It's not only up to the men to change their ways. The women are to blame as well. Should be a united effort. And I believe it starts with the family unit.