It is a rarity in the life of a sports fan to be able to stay with a single star player and have him or her be there throughout the major events in both of your lives. NBA stars jump from team to team either during their youth or to chase championships in their later years, but players like Tim Duncan have remained constant.
New teams offer changes of scenery, new teammates, opportunities outside of basketball and on occasion, more money. Those are the major reasons for players choosing the free-agency route and moving from city to city.
Another cause of players switching jerseys is trades. The NBA is very much a business and loyalty is not often valued by either players or front offices. This leads to players being swapped, sometimes with regularity.
It is a different kind of honor to play out a full career in one place. An honor that differs from individual awards or championships. This is something that can never be discredited or debated. Staying in one place for an entire career means always having a second home where you will forever be a hero.