There are a lot more great QBs who went into terrible environments and turned the environment around than there are great QBs who came into a bad environment, failed, then succeeded later in a good environment.
It's a feedback circle. When a QB comes in and does well, the coaches look smart, the support players put up better stats, everyone says "oh he had a great environment."
When a QB sucks, the coaches look like idiots, the offensive line gets blamed, the support cast sees their stats fall apart. "Oh no one could succeed in that environment."