Why couldn't she...

She did run off once during a snow storm...didn't get very far. She could have done or attempted more. I agree she was too passive...maybe she was written to be more passive so that the plot could play out like it did.
And weird that there would be a phone in the attic. If the 2 men were more prepared, they would have not let the woman find the pictures or have access to a phone or see her ID getting burned up. But there has to be flaws in the characters or it wouldn't be interesting.

Also, remember, they led her to believe the car wouldn't start. The butler sneaked off while she was asleep to send the tape. They were in a very isolated area during a bunch of storms, so even though she did attempt to escape on foot, it was not very likely that she would have made it.

They also kept drugging her. And she did eventually overpower all of them.