1. Most boys learn to hide their emotions, so the fanboys you notice are bad at holding it all in.
2. Society has taught boys that being hooked on a book or movie embarrassing, so they don't exactly announce they are "Dweebs" or "Nerds", when it is not only perfectly normal to read, but healthy.
3. Harry Potter fanboys are very common, but thanks to a entertainment company that shall remain unnamed (*cough* Disney *cough*), Potterheads, Muggle Quiddich players and fans, have been portrayed as nerds with absolutely no social life, and in one episode, the fan in question was a total psychopath who was a bloodless yandere! *shudder* This is absolutely untrue.
4. Most fangirls don't look for fanboys, thinking they are to rare to find. If Voldemort thought the Sorcerer's Stone was to rare to find, would we have a series?