That's because alot of the team specific playbooks are terrible. There are plays in Carolina's playbook that they never run, and they're missing formations that they run often. It's common knowledge that some playbooks are far better than others. Why would someone limit themselves to one PB when they're trying to compete.
Playbooks have zilch to do with being SIM as users play to their strengths more so than a teams strength, so why pick a running playbook, when you're more of a passer. That's narrow minded thinking to consider swapping playbooks un-SIM.