They've never been outside in their lives nor have they ever been given a choice to do so. They have been surrounded by wire their entire lives and have no idea what they are capable of doing.
Coop bound hens is a common problem among these rescued gals. Once you add them to the existing flock they will get the idea. However, if there is no existing flock to teach them how to act (or are in quarantine) you will have to be their teacher. Don't try to starve them to get them to go outside. Right now they have no idea how to forage for food. Food has always been right in front of their face and they are not capable of thinking on how to find food at this point. Don't worry, though, they'll figure things out.
When the weather is halfway decent you can place the hens outside with food and water near by in a pen for a short period of time. For the first few times I'd stay with them to make sure they don't get into trouble. Your gals still have certain instincts and may panic if a cloud passes over head. Your gals won't know how to navigate a ramp, nor will they attempt to move from their comfort zone. That's okay they will eventually figure out they can actually move without running into wire or another chicken.
Don't despair! Because once these gals get over their fear, and realize they are now 'real' chickens, they will start to act like chickens. When they learn they can walk on grass, you can toss their food in front of them so they will get the idea on how to forage. Be patient, once they feel safe they will start acting like chickens.
And if you do plan to add them to your existing flock, watch them as they can get picked on. They lack the social skills to navigate flock dynamics and may need extra care for a few weeks until they figure out how to behave appropriately among other ladies.
So gratifying to watch these timid creature slowly blossom to real, living creatures. Congratulations!