Here's why:
* Technology: The technology in the movie, particularly the computers and video game consoles, looks very similar to what was available in the early 2000s.
* Music: The music played in the film, especially the soundtrack, features pop songs popular in the early 2000s.
* Fashion: The fashion choices of the characters, especially Sulley and Mike, reflect clothing styles that were popular around this time.
* Setting: The film takes place in a city built on the back of modern energy production, fitting with the energy concerns of the early 2000s.
Ultimately, there is no definitive answer, but the movie's design and references suggest a setting sometime in the early-to-mid 2000s.