However, leaving does cost something. In fact it will cost quite a lot.
Consider what is left behind when someone leaves:
There is a good chance she will be leaving every physical item in her home,
Most if not all of her friends that are friends with both parties,
Her partner is probably her closest friend and she's leaving that behind,
The security of knowing what comes next, sometimes the unknown feels worse,
She is leaving behind the dreams of her future, they probably didn't include being alone,
Married people carry a different status in our society, and someone leaving a long term partner is leaving a status.
She could be leaving her job,
If moving is required then she is leaving all the securities of home,
She is probably leaving the security of having two incomes to depend on.
Often she won't know how to do basic home or auto repairs without help,
She will need to change predictable patterns, so she will be leaving those daily comforts behind,
In many instances she is walking away from her children or family members
and she is facing a court system that may or may not understand what she is going through.
Those are just the initial reasons 'why', but it doesn't begin to touch the deeper aspects of what 'just leaving' really means. Often women who leave will decide to head into shelters for some measure of protection, but research shows only about 6% of survivors will go to a shelter setting. Most will lean on friends or close family during the crisis of leaving, this temporary support can and does help, but these women are still looking at living with room mates they depend on. Often they are stressed in addition to adjusting to a new community, trying to figure out child support and parenting plans. Many meetings to meet the requirements for assistance are in different locations and can be exhausting to navigate. To explain why they need the benefits for a short time, usually requires openly sharing intimate aspects of their lives with strangers or attorney's. There are financial costs involved and issues such as leaving behind pets can be an emotional cost. If they have nothing, then simple needs like transportation or adjusting to new schools can be literally un-affordable for a period of time.
Leaving is somewhat like starting a new life at the drop of a hat and its not always easier than putting up with a few arguments. The arguments have a cost too, but at least when staying the victim has a general idea of what that cost looks and feels like. Even with the number of things someone is facing when they leave.
"Just leave" shouldn't even be a phrase used when talking to or about an abused individual.