The Marvel Cinematic Universe demands sacrifice. Not anything approaching blood or your immortal soul, but not far off. When an actor gets cast in an MCU film, nine times out of ten it's required that they sign on for multiple movies - Scarlett Johansson has appeared in both Avengers, Iron Man 2, and is still due to appear in the third Captain America film after stealing the show in The Winter Soldier. In that case, why not cast from within the existing roster of MCU actors for Peter Parker? They already know the deal, and it'll simply be a matter of renewing an existing contract; not only that, but Marvel can pretend that Spidey always was part of their Cinematic Universe, rather than only when Sony allowed it. Simpkins is 13 years old - that's around the high school age they're shooting for, and presumably it'll be a while before the new Spider-Man film actually enters production - and, besides being the creepy kid from the Insidious franchise, he's probably best known for being the precocious kid Tony Stark hung out with for most of Iron Man 3. Who was smart. And was at least semi-orphaned. Hmm...
Tom Baker is the Comics Editor at WhatCulture! He's heard all the Doctor Who jokes, but not many about Randall and Hopkirk. He also blogs at http://communibearsilostate.wordpress.com/