Thanos Thanos:
Because things like this are usually accompanied by massive amounts of wife beating, marital rape, threats to leave them in poverty or to even have them killed, and to have them permanently shunned by the entire community which would sever all their ties to family and friends. The women and girls (because an appallingly large number of them are underage) are intimidated into compliance from the beginning and remain terrified enough that they're paralyzed into inaction. This is no different than what a bastard like Warren Jeffs tried to pull off with his polygamous Mormon cult in the US. These Muslim males engaging in this same crap deserve to go to prison for it as much as Jeffs did.
That's absolutely a concern, which is, again, why we should over-rule any extra-judicial outcome that has in it a violation of our laws. But you don't throw the baby out with the bathwater. Your use of "usually" is untrue. Usually these cases involve simple, private disputes that are resolved in manners perfectly consistent with our laws. Your opening statement, were it truthful, would go like "Because these things are rarely accompanied by..." we need to set some rules. A lot of the problems you identify, rightly, come from the fact that these hearings are done under cover of dark. But if we allowed them, and made their decisions subject to judicial review, those problems could be identified and combated. As it is, Sharia happens. Its users just don't broadcast it too much. Get the whole thing out into the light and then maybe things will be better for Muslim (and Jewish and Catholic) women.