Also correlates with free agency. If you built an elite offense or defense back then, you got to keep it. Well once that happens, who knows how superior you are to the other coaches Xs and Os wise? 10 years are gonna need to go by until we see you with a different team. Like Chuck Knoll in the 80's. So someone like Gibbs, as long as he leaves at the right time, never seen with makeup off (until his weakass comeback in 2004)
Also, game was still just in its infancy back then from a schematic perspective. The edge between someone that knew what they were doing vs someone that didn't was just a lot bigger. Nowadays every team is owned by a billionaire with a massive scouting/analytics operation, tougher to get a leg up tactically speaking.
It was actually Gibbs that dealt Champ Bailey for Portis, awful trade that doomed him from the jump.