Seems to be this strange phenomenon in American sports where players are designated as starters and if they don’t start for any reason other than injury it becomes a huge deal

Let me take a crack at it. This is just a personal theory. There is a hierarchy driven by toxic masculinity in American football that is tough to escape. QBs are are the top of the food chain. They've been conditiond in high school and college to be the alpha male and most don't have the emotional capacity to deal with sharing their spotlight with another. Once somebody is determined to be "the man" it would be too great of a shock to the hierarchical system to diminish their value and share it with another. This creates a lot of pressure on the coaches to keep from rocking the boat too much. It's also very political. If you look into high school football you'll see how football is a religion to many areas and powerful family units have a lot of influence where their own are protected. The jobs and starting positions are scarce and the most powerful keep the jobs. The idea of "the franchise QB" aka the most powerful position in football being replaced with another is just so abnormal within this structure our society has influenced. Some of this is generalization and not every player or coach fits this theory but the overwhemling majority have over the years. I also could be full of shit I don't know.