introvert giving up on relationships

So my 17.5 y/o daughter has introvert tendencies and tends to isolate herself from family and has kind of given up on making friends. We homeschool and are involved in a large co-op where she does have some on-the-surface friends, but none that she seeks out or who seek her out. Same with our church youth group. She interacts fine when there is an activity or class going on, so she is not shy or socially awkward, but when there are informal opportunities to interact, she often will choose to read a book instead of interacting with people. She is an addictive reader, and has said that fantasy is so much better than reality, which reinforces her self-isolation, and feeds her lack of desire to socially interact because her fantasy world is better in her mind.
When I talk with her about this, she gets annoyed that I am bringing this up…again, because she just doesn't care anymore and has made up her mind about it. She says that she used to feel depressed because she didn't have any close friends and she was trying to break into social circles, but then about a year ago, she gave up trying to do this, and instead just accepted herself for who she is, and says that she is much happier now. However, I think this is just a cop-out to give up on all relationships, because I do believe she could make friends if she would decide to and make an effort.
She often does the same at home by isolating herself with her books and not putting any effort into her relationships with her three younger sisters (15, 14, 12 y/o). My 3 younger girls are all really close with each other, but not with my oldest daughter because she usually chooses to not engage with them. She is close to her older brother, who is now serving a church mission, so she is not able to interact with him much right now.
She sometimes will admit that she is lonely, but will other times say that she just doesn't do relationships, or that relationships don't work for her, or that she's introverted, and so that's just how she is. It feels to me that she has labeled herself as an introvert and is using it as an excuse to not interact or to rationalize her escapism (reading books). This is her last year at home, and I'm concerned about what she will do when she goes off to college, and I am hopeful she can learn how to initiate and foster relationships now, but it is hard when she doesn't engage with anyone. This is also the same child I asked you about last week who has an aversion to goals.