No studio in their right mind should give newbies a chance to direct blockbusters. You don't let a pilot that has never flown a plane before start flying jumbo jets frequently. They need to build up their skills, and they do that by acquiring experience. That goes for any profession or goal for that matter. Start small, aim big.
A lot studios go for exactly that nowadays, to great success
The Russo Bros were pretty damn new / inexperienced. Their only major film direction role was You, Me, and Dupree. They had really only otherwise directed Arrested Development and Community episodes.
Jon Watts is directing Spider-Man Homecoming, it's a huge deal to be trusted to headline a major franchise like that. He has directed a low budget, trashy horror movie and an indie flick starting Kevin Bacon
It goes further than Marvel Studios too.
The Amazing Spider-Man franchise with Marc Webb
Kong: Skull Island with Jordan Vogt-Roberts
Collin Trevorrow directed Jurassic World and will be directing Star Wars Episode 9. He has two sub $1m budget Indies under his belt and a short film.
Snow White and the Huntsman - Rupert Sanders, it was his first feature length job. His next job? Ghost in the Shell
It's actually now odd to hire a very opinionated, established​ director like James Cameron or Christopher Nolan for a big franchise film. It's easier for producers to manage a small director instead.
Someday they might cut out the middleman and just have the cinematographer, producers, and second unit director all collaborate to fill the role of "Director"