I might be risking giving out the secret but the main reason is the shallow guy is telling them what they want to hear, at least initially. Such girls do not have a good support system home that teaches them self-worth and what they should aim for.
Poster above is correct and not only that they should be hearing those things at home but sometimes they don't so they like the sound of it from the "Shallow" guy but what they don't know is the words sound good but there is nothing behind those words. Its just a deception to get in your pants.