In "The Great Gatsby," why does the reporter go out to West Egg "to see" about Gatsby?
There is no concrete reason for the reporter to visit Gatsby. Nick tells us that the man came, compelled by a "reporter's instinct." The reporter is attracted by Gatsby's "notoriety, spread by hundreds who had accepted his hospitality and so became authorities on his past." Like all rumors, this one had gotten out of hand and fantastical, like the "one persistent story that he (Gatsby) didn't live in a house at all, but in a boat that looked like a house and was moved secretly up and down the Long Island shore."
The people who claim to "know" Gatsby, of course, really don't know him at all. Gatsby himself has been an enabler in their fantasies, and had been for numerous years. As Nick relates the real history of the mythical Gatsby, we learn that the "truth was that Jay Gatsby, of West Egg, Long Island, sprang from his Platonic conception of himself...So he invented just the sort of Jay Gatsby that a seventeen year old boy would like to invent...".
Gatsby puts on a show, the revelers come to watch, but it is all an illusion.