If the Russo Brothers all of a sudden made Spider-Man a mature warrior in the face of Thanos for Avengers: Infinity War, it would contradict all that Watts is trying to establish in Homecoming.
The MCU thrives on continuity, so it’s important for filmmakers like Jon Watts and James Gunn to have a say in how their heroes are treated in the grand Avengers game plan.
Tom Holland’s Peter Parker is a high school kid, and subsequent Spidey sequels are expecting to keep him in high school, where his innocence essentially informs his character.