Rey expects to see who her parents are. However, instead, she simply sees a reflection of herself. As Rian Johnson explains, the Cave is a twist on Luke's experience on Dagobah. Luke's most significant challenge was accepting his heritage and how he was associated with - and could even become - Darth Vader.  For Rey, her biggest obstacle is not knowing who her parents were. The Cave shows her the problem she needs to overcome - not overcoming it for her.