It’s the same situation that gave us the animated series The Batman and Rise of the TMNT. Sony had to create a movie that was still Spider-Man, but different enough to not be the previous Spider-Man. Five years after we saw a defining take on the hero. A reboot five years after his last adventure. Fine, Sony more or less announced, they would have the reboot ready for summer 2012. Hence, the studo made it clear that Spider-Man was getting a reboot in the same week Raimi announced he would not be able to get Spider-Man 4 into theaters by 2011.
Not to mention that they still had to crank out Spider-Man movies every few years to keep the film rights. Raimi was done, but money showed that people still wanted motion pictures of Spider-Man. Sam Raimi called it quits early in production on Spider-Man 4 and none of the other main actors cared to stick around either.