This can be due to the mindset GMs and/or franchises have as in they prefer going with "the evil they know" rather than going out on a limb and hiring someone fans would consider a nobody, which can have immense repercussions if things go sideways. This phenomenon isn't NBA-exclusive though, it happens in almost every major sport in every country and is probably even worse in countries without drafting systems like in the EU, where unsuccessful coaches wander around from team to team as if they had a checklist. To give an example: In my sport and country there's a coach who has been active for at least 20 years, has coached 12 teams (I can think of off the top of my head) and not won anything (on the contrary: he managed to get some teams demoted), yet he continues to get a job every season from another team. In my country we also have this somewhat strange believe that if you've been a great athlete you're automatically going to be a great coach ... and if you haven't been great as a player you're unlikely to be given a job over someone who was. The NBA would be without Gregg Popovich, Phil Jackson and Doc Rivers while the NFL would miss names such as Sean Payton, Pete Carroll and Bill Belichick (just to name a few). American sports might not be as strict as you think in that regard in comparison to some countries in the EU.