That sounds like one of two things.
A) It’s a cover because she simply doesn’t have romantic feelings for you. IF thats the case, you can’t do anything really. That has been known to “blossom” into a romantic relationship, but there’s nothing you can do to make it. In fact pushing too hard could very easily mess up the friendship.
B) She doesn’t want to be tied down to someone that would be a long term relationship or potentially a marriage partner. This would be especially more likely if she’s young (Teens/early 20’s). At that age a lot of people will mess up good relationship opportunities (even subconsciously so) simply because they’re not ready to settle down, and deep down they know that a person might be a long term relationship. What you could try to do in this case (though it would be very hard and again could mess up the friendship if you push too hard) is make her see that being in a long term relationship isn’t such a bad thing. Most people NEED to go thru a phase of not being tied down though, and won’t know that they’re happier with someone til they’ve been thru it. Unfortunately many amazing relationships have never been because of that need (and that’s just speaking from my own experience :P )
(But don’t think it couldn’t be something other than these things, every girl is different…. just my two cents)