In my opinion a person needs someone to care about. They want to be loved, obviously, but they also want someone to love. There are times in a persons life when they are not into the person they are with or they break up with someone and are at an in-between point in their life where they have no one to love. So they start crushing on a celebrity. Or maybe they just always have a crush on a celebrity because celebrities are easy to crush on. Anyways, they are the perfect crush because (a) they are always GORGEOUS, (b) you're never going to meet them so you can imagine they have the perfect persona to fit what you need, and (c) since you are never going to actually meet them they will never let you down.
A celebrity crush is the perfect escape from reality. Especially because reality isn't always as joyous and as perfect as you want it to be.