They could have set the movie years after the events of Captain America: Civil War, but doing so would have put Peter out of high school or close to it, and it’s clear, based on these comments made during a Spider-man: Homecoming set visit by our own Sean O’Connell, that they see the high school age of the character as the key differentiation between the MCU version of Spider-Man and every other version that we’ve seen over the last few years.
Ultimately, Eric Carroll shows just how important it was to Spider-Man: Homecoming that Peter Parker be a high school student, though he does exaggerate how long it’s been since we last saw the Spider-Man character on screen.