Please don't give up on this girl; you CAN teach her, although it will take time and patience. Keep in mind that she lived in a small cage almost all her life and her brain has to "undo" years of having no other choice but to "go" in the cage.
I'm sure the shelter has started to work with her on house training but most likely she is one of the many dogs they care for and does not benefit from a lot of "one-on-one" time.
Once you bring her home and establish a routine it's going to be much less complicated for her to understand what she needs to do. It will take time, but with patience and lots of tasty treats
she will "get it".
Don't expect overnight miracles and give her time to accept that her nightmare is over and will have a LIFE in your home.