thats because people are, in general, stupid...
"crushes" on celebrities is more based on EVERYONE else loving them as opposed to a person actually liking them. (examples: Tom cruise, Brad Pitt etc. etc. as...well girls are more prone to this syndrome then guys are) they "like" these people because they are "told" to like them.
plus its illogical to have a "crush" on someone you've never met before.