TL;DR: because you need professionals for modern CG and programming, professionals deserve good pay and you need a lot of them, also considerable marketing is required for non-innovative games.
The real reasons are management overhead and big corporations viewing all developers as equivalent, expendable resources even though it leads to subpar work, extended deadlines and constant industry burnouts. Technology getting more complicated is a poor excuse for the AAA game industry's gross incompetence in mismanaged funding.