Trivia: The pic of Spidey on Harry's desk is very similar to the one seen on the cover of the first Spider-Man comic.
Trivia: Co-writers Alfred Gough and Miles Millar are reportedly responsible for much of the final script, though they only received a story credit due to the confusing rules and studio red-tape regarding how screenwriters are credited.
Trivia: The Blues Brothers director John Landis has a cameo as the doctor who is killed by Dr. Octopus.
Trivia: Director Sam Raimi and composer Danny Elfman had a major falling out during production, and Elfman vowed never to work with Raimi again. As a result Elfman didn't return to work on "Spider-Man 3." However, a few years later, Elfman and Raimi made up, became friends again and ended up working together on several more films, including 2013's "Oz the Great and Powerful" and the upcoming "Doctor Strange in the Multiverse of Madness."
Trivia: Michael Buble sings the Spider-Man theme in the credits.
Answer: While it's never mentioned in the film, the most likely explanation is that Harry, as a rich kid, grew tired of "slumming it" and decided to move back into his mansion.
Harry and Peter's relationship quickly turned sour after Peter refused to discuss Spider-Man as Harry felt Peter knew more than he let on. Peter is Spider-Man's "personal photographer," and Harry only wants to avenge the death of his father. It's not unlikely to assume Harry left to live alone because he was angry Peter wouldn't help him find Spider-Man.