I wonder how much of that belief stems from cultural institutions where top positions at the best companies use good schools and good grades as primary hiring criteria.

In the US, that is still very much the case at a lot of companies outside of hiring developers in the tech industry. Can't say I entirely blame them as much as I dislike the practice. When someone lacks experience because they are entry-level, that is pretty much the next best indicator you have as to some level of intelligence and drive to succeed.