Much of the mentality is GM/Front Office driven, where they believe they need to win in the first four years after the QB has been drafted (where their cap hits are much lower), before they need to give the QB a lucrative contract and hamper the cap in year 5 and beyond. Not saying I agree with their logic but that's what I hear a lot of analysts talk about.
Having said that, some qbs take time (a yr or two, e.g. Montana, Esaison, Rivers, Rodgers, Simms took a real long time), but many of the greats were thrown right into the mix to start year 1 of their careers. Aikman, Marino, Elway, Peyton Manning, Roethlisberger, Russell Wilson, Jim Kelly, etc..