During the weekend press for Spider-Man: Homecoming, Kevin Feige revealed to io9 why Homecoming sequel will be perfect to start the new Marvel Cinematic Universe after the fourth Avengers film. He said:
“So much happens in [the 3rd and 4th Avengers movies], as you can imagine, and so much is affected by it that we felt what better person to hold your hand and lead you into the next incarnation of the MCU, in a grounded, realistic manner, than Peter Parker? So, coming out two months after Untitled Avengers, [that’s what] much of what the next Spider-Man film will be about.
It was purposefully done like this. Like how the events of Captain America: Civil War impacted Peter as he was dropped off by Tony and expected to go back to continue his sophomore year, how the hell are the events of Infinity War and Untitled [Avengers] going to affect him as he, yes, goes back to his junior year?”