Because they're not planning to do just one movie with him. I mean, look at RDJ. He's been Iron Man almost 10 years now and he's getting old as fuck and they keep constantly having to renegotiate to keep him.
Or look at DCEU Batman. They decided to do Dark Knight Returns in his first fucking movie, so by the time we get to Batman 3 it's going to strain credulity that he can still keep fighting in his 70s.
Starting with young Spider-Man gives them a chance to keep him for many movies and gradually age him as they go.